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ORIGIN  OF  THE  ELECTRIC  TISSUES  OF  GYMNARCHUS 
NILOTICUS. 


By  ulric  dahlgren 

Professor  of  Biology,  Princeton  University. 


Nine  plates  and  nine  text-figures. 


[Extracted  from  Publication  No.  183  of  the  Carnegie  Institution  of  Washington, 
pages  159-194.     1914. 


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http://www.archive.org/details/originofelectricOOdahl 


ORIGIN  OF  THE  ELECTRIC  TISSUES  OF  GYMNARCHUS 
NILOTICUS. 


By  ulric  dahlgren 

Professor  of  Biology,  Princeton  University. 


Nine  plates  and  nine  text-figures. 


7>I3 


5f2 


ORIGIN  OF  THE  ELECTRIC  TISSUES  OF  GYMNARCHUS  NILOTICUS. 


By  Ulric  Dahlgren. 


In  the  seven  types  of  electricity-producing  fishes  the  exact  development 
by  which  the  electric  organs  and  tissues  are  produced  during  the  creature's 
life  is  known  in  only  two,  leaving  the  other  five  unknown.  It  also  happens 
that  the  two  forms  which  have  been  studied  as  to  the  histogenesis  of  their 
electric  organs  are  the  only  two  elasmobranch  fishes  among  the  seven,  so  that 
we  have  not  as  yet  seen  how  the  remarkable  electric  tissues  in  Malopterurus, 
Gymnotus,  Astroscopiis,  the  mormyrids,  and  Gymnarchus  are  developed.  Also, 
of  the  five  teleost  types  we  know  the  structure  of  the  full-grown  electric  organs 
in  all  of  them  pretty  well,  except  in  Gymnarchus.  This  fish  is  found  in  Africa 
and  has  been  rather  rare,  so  that  but  two  workers  have  published  observa- 
tions on  it,  both  of  them  a  long  time  ago  and  from  poorly  preserved  material. 

It  was,  therefore,  with  great  pleasure  that  the  writer  came  into  possession 
of  some  embryos  of  this  rare  form  through  the  kindness  of  Dr.  J.  Graham 
Kerr,  Dr.  Arthur  Shipley,  and  Dr.  Richard  Assheton,  to  whom  he  wishes 
to  express  his  most  sincere  thanks.  This  material  was  collected  in  Africa 
by  Dr.  Samuel  Budgett  some  years  ago  and  was  in  most  excellent  condition, 
owing  to  the  great  care  and  skill  with  which  Dr.  Budgett  put  it  up  and  cared 
for  it.  The  collecting  was  done  unflinchingly  and  faithfully,  under  con- 
ditions of  hardship  and  sickness  that  few  white  men  could  stand,  and  Dr, 
Budgett  lost  his  life  from  exposure  and  illness  incurred  in  part  by  this  work. 
A  full  account  of  his  trip  and  of  the  scientific  results  should  be  read  in  the 
Budgett  memorial  volume  issued  by  Dr.  Shipley,  Dr.  Kerr,  Dr.  Assheton, 
and  others  in  1907,  through  the  Cambridge  University  Press  (24).' 

It  is  somewhat  unfortunate  that  the  structure  of  the  electric  organs  in 
the  adult  fish  could  not  be  worked  up  at  the  same  time  that  this  paper  was 
written,  but  the  writer  has  material  on  the  way  from  Khartoum  and  hopes 
to  publish  a  second  paper  shortly. 

But  three  papers  have  been  published  on  the  electric  organ  of  this  inter- 
esting fish,  one  by  Erdl  (15)  in  1847  and  another  by  G.  Fritsch  (19)  in  1885. 
Riippel's  publication  on  the  subject  could  not  be  found,  but  Fritsch  states 
that  Riippel  mentioned  the  peculiar  structures  which  we  are  considering, 
and  so  he  stands  at  present  in  the  writer's  knowledge  as  the  first  one  to  have 
seen  and  reported  to  science  the  electric  organs  of  this  fish,  although  he 
was  in  doubt  as  to  their  significance.     Erdl  used  a  specimen  which  was  so 

^  The  figure3  in  parenthesis  refer  to  the  literature  cited,  p.  193. 


I62 


Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 


poorly  preserved  and  so  soft  that  when  he  cut  the  animal  across  its  body, 
the  electroplaxes  and  connective  tissue  ran  out  of  the  muscle  in  which  they 
are  embedded.  Nevertheless  Erdl  stated  that  it  was  probably  an  electric 
organ,  coming  to  this  conclusion  by  a  comparison,  of  such  features  as  he 
could  make  out,  with  the  structures  found  in  the  tails  of  the  other  electric 
fishes,  as  Raja,  Mormyrus,  and  Gymnotus. 


Fig.  I. — General  view  of  Gymnarchus  nilolicns.  (Drawn  from  a 
lantern  slide  made  from  a  figure  in  Jordan's  "Guide  to  the  Study 
of  Fishes,"     New  York,  190s.) 

Fritsch  (19)  in  1885  worked  on  better  material  and  gave  a  more  com- 
plete account  of  the  anatomy  of  the  organ,  especially  of  the  histology  of  the 
electroplax;  but  he  came  to  the  rather  strange  conclusion  that  it  was  not 
an  electric  organ  at  all,  assuming  the  erroneous  position  that,  on  account 
of  the  large  blood-supply,  the  organ  acted  in  some  way  as  a  storage  for 
oxygen  during  the  period  of  hibernation  made  necessary  by  the  drying  of 
waters  at  certain  seasons.     Fritsch  was  also  mistaken  in  calling  the  fibrous 


Fig.  2. — Scene  drawn  from  descriptions  to  illustrate  the  habitat  of  Gymnarchus  and  its  manner 
of  swimming.    (Drawn  from  lantern  slide  made  from  a  drawing,  by  Bruce  Horstall.) 

contents  of  the  electroplaxes  "connective  tissue."  He  went  a  great  deal 
farther  than  Erdl,  however,  in  describing  the  gross  anatomy  of  the  electric 
organs  and  surrounding  tissues. 

Not  having  full-grown  material,  the  writer  must  rely  on  Fritsch's  figures 
and  descriptions  for  the  adult  gross  anatomy,  although  the  oldest  embryos 


Origin  of  Electric  Tissues  of  Gymnarclius  Niloticus. 


163 


Fig.  3. — Section  through  mid-tail  region 
of  body  of  an  adult  Gyjmiarchus, 
showing  position  of  the  eight  electric 
spindles  and  their  relations  to  sur- 
rounding muscular  and  bony  struc- 
tures. D,  dorsal  spindles;  U.M„ 
upper  median  spindles;  L.M.,  lower 
median  spindles;  F,  ventral  spindles. 
(After  Fritsch.)      X  unknown. 


(about  42  days)  used  for  this  paper  represent  practically  adult  material 
so  far  as  the  histology  of  the  electroplaxes  is  concerned.  A  short  resume 
of  the  adult  anatomy  will  make  a  good  basis  for  the  embryological  descrip- 
tions to  follow. 

The  fish  (see  text-fig.  i)  is  a  mormyrid  of  elongate  form,  so  much  so  as  to 
make  it  almost  eel-shaped,  although  not  quite  so  much  so  as  the  Gymnotus 
of  South  America.  It  possesses  an  extensive  development  of  the  dorsal 
fin,  which  extends  from  forward  on  the  neck  to  within  a  short  distance 
of  the  tip  of  the  tail,  where  it  suddenly  stops,  leaving  the  tip  of  the  tail 
naked  of  fin;  whence  the  name  of  the  fish,  Gymnarchus  niloticus.  This 
heavy  fin,  well  provided  with  a 
series  of  lateral  ray-muscles,  is 
used  extensively  by  the  fish  as  a 
means  of  propulsion,  by  holding 
the  body  straight  and  stiff  and 
causing  a  series  of  lateral  undu- 
lations to  pass  from  behind  for- 
ward, thus  driving  the  body  bsxk- 
ward  (see  text-fig.  2)  or,  from  front 
to  rear,  which  causes  the  fish  to 
move  with  its  head  forward.  I 
have  not  heard,  but  I  presume 
that  in  moments  of  unusual  effort 
the  animal  can  swim  by  means  of 
the  common  sinuous  body-move- 
ments used  by  other  elongate  fishes, 
as  is  well  illustrated  in  the  eel. 

The  posterior  tip  of  the  creature's  body  is  interesting.  As  has  been 
mentioned,  this  end  is  free  from  the  fin  for  some  distance  (see  text-fig.  2). 
Also  it  is  round  in  section  and  ends  bluntly.  When  swimming  backwards 
the  animal  uses  it  like  a  finger  to  feel  its  way.  The  peculiar  round  and 
blunt  end  may  be  explained  by  the  fact  that  this  tip  contains  the  largest  and 
best-developed  portion  of  the  electric  organ,  which  fills  the  lateral  parts  of 
the  body  at  this  point  almost  to  the  exclusion  of  the  ordinary  muscle. 

As  Erdl  and  Fritsch  have  described,  the  electric  organ  consists  of  eight 
long  "tube-like"  or  cylindrical  structures,  four  on  each  side,  embedded  in 
the  muscle  tissue  as  close  to  the  median  bony  parts  as  a  little  connective 
tissue  in  between  will  permit.  Four  of  these  are  present  on  each  side 
(see  text-figs.  3  and  4),  and  they  may  be  called  in  order  from  above  down- 
ward, the  dorsal,  the  upper  middle,  the  lower  middle,  and  the  ventral  cylinders, 
or  spindles,  of  the  electric  organ.  In  a  section  cut  through  the  body  at  a 
point  midway  between  tail-point  and  anus  (see  text-fig.  3)  the  dorsal  spindles 
are  to  be  found,  just  above  the  union  of  the  neural  spines  of  the  vertebrae, 
and  set  closely  together  with  only  the  dorsal  spine  and  some  connective  tissue 
between  them.     The  upper  median  spindles  are  more  widely  separated  by 


1 64 


Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 


the  neural  canal  and  its  contained  spinal  cord.  This  second  pair  of  spindles 
are  at  the  level  of  the  upper  part  of  the  spinal  cord.  The  lower  median 
spindles  are  found  much  below,  on  each  side  of  the  ventral  processes  of  the 
vertebra.  They  lie  at  about  the  level  of  the  caudal  artery  and  almost  as 
widely  separated  as  the  upper  median  pair.  Lastly,  the  ventral  pair  of 
spindles  are  found  just  below  the  latter  pair  and  slightly  below  the  level  of 
the  caudal  vein,  which  lies  between  them.  They  are  separated  a  little  less 
than  the  lower  median  pair  by  the  narrower  bony  structures  at  this  point. 


,Upper  median 
Dorsal 


Ventral    Lower  median 


Fig.  4. — Diagram  showing  longitudinal  position  and  extent  of  electric  spindles  in  Gymnarchus. 
Those  of  one  side  only  can  be  shown  in  figure. 

The  four  spindles  are  largest  in  diameter  in  the  tail,  especially  out  in  the 
thick,  finger-like,  naked  extremity.  From  this  part  they  taper  to  a  smaller 
size  as  they  go  forward  in  the  body  and  they  finally  become  thin  and  end  at 
points  in  the  neighborhood  of  the  anal  opening  (see  text-fig.  4).  All  are  not 
of  equal  length.  As  Fritsch  has  shown,  the  dorsal  organ  is  the  shortest  and 
extends  for  about  20  cm.  in  a  fish  of  89  cm.  length.  The  ventral  pair  reach 
for  about  5  cm.  further,  or  25  cm.  in  length  in  a  fish  of  the  same  size.  The 
two  median  spindles  reach  for  about  40  cm.  from  the  tip  of  the  tail. 

Each  spindle  is  marked  off  clearly  from  the  neighboring  muscle,  and  other 
tissues  which  are  found  next  to  it,  by  a  distinct  connective-tissue  covering. 
In  my  largest  specimen,  which  is  a  young  fish  of  40  days,  this  is  well  shown 
and  is  exactly  like  other  dividing  connective-tissue  sheaths  that  surround 
the  various  muscle  divisions.  Like  them,  it  often  contains  pigment  cells 
which  show  golden-brown  pigment  granules. 

The  important  contents  of  these  spindles  are  alternate,  cylindrical 
segments  of  a  denser,  deeper-staining,  muscle-like  substance,  the  electro- 
plaxes;  and  a  connective  tissue  of  jelly-like,  grayish  transparence  which  in 
all  ways  appears  to  be  similar  to  the  "electric  connective-tissue"  found 
between  the  electroplaxes  in  the  other  electric  fishes.  In  this  tissue  are 
found  the  blood-supply,  which  is  largely  in  contact  with  the  ends  of  the 
electroplaxes,  principally  the  anterior  end;  and  the  nerve-supply  of  thick, 
medullated  fibers  which  run  towards,  and  are  attached  to,  the  posterior 
ends  of  these  organs.  In  this  case,  according  to  both  Pacini's  law  and  the 
fish's  relationship  to  Mormyrus,  the  direction  of  the  current  at  time  of 
discharge  should  be  from  tail  towards  the  head.  I  have  been  unable  to 
learn,  from  the  literature  of  travelers  and  scientific  collectors  and  observers, 
if  the  electric  discharge  is  strong  enough  to  be  felt  by  the  hand.  Budgett 
does  not  mention  it,  and  no  other  does,  so  I  conclude  that  it  is  not  a  strong 
shock  and  that  the  organ  must  be  classed  with  the  weak  electric  organs, 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus.  165 

as  is  the  case  with  its  relatives,  the  various  mormyrids.  The  natives  of 
Africa  are  much  afraid  of  the  creature,  especially  at  nesting  time,  and  one 
of  its  Arabian  names,  "Abu  rhad"  meaning  "father  of  thunders,"  might 
seem  to  indicate  perceptible  electric  powers. 

The  embryos  and  young  fishes  put  at  my  disposal  by  Dr.  Kerr  and  Dr. 
Assheton  were  five  in  number,  and  of  these  three  were  the  suitable  stages 
from  which  this  paper  was  worked  out.  The  significant  development  of 
the  electric  tissues  in  Gymnarchus  takes  place  between  the  ninth  day  of 
embryonic  life,  at  which  time  the  embryo  possesses  a  fully  formed  and 
complete  musculature  in  the  tail  with  no  sign  of  an  electric  organ,  and  the 
fortieth  day  of  development,  at  which  time  it  can  be  seen  that  the  embryo 
has  developed  its  electric  organs,  out  of  a  certain  part  of  the  previous  muscu- 
lature in  the  tail,  to  a  degree  that  shows  the  farthest  advanced  electro- 
plaxes  in  a  practically  adult  condition. 

The  most  interesting  and  critical  stages  in  this  metamorphosis  of  muscle 
into  electroplax  appear  to  take  place  within  much  closer  limits,  and  stages 
from  the  eleventh  to  the  fifteenth  day  would  include  them.  These  signifi- 
cant changes  have  been  studied  and  drawn  principally  from  an  embryo 
12  days  of  age,  fixed  in  sublimate-acetic,  and  showing  the  changes  very 
much  to  my  satisfaction.  A  point  of  interest  and  importance  in  this  study 
is  that,  in  earlier  embryos,  the  myotomes  and  electric  spindles  are  youngest, 
least  developed,  and  growing  fastest  in  the  posterior  part  of  the  body  or 
nearest  the  tip  of  the  tail;  while  in  older  embryos  and  in  the  adult  the 
greatest,  most  complete,  and  most  characteristic  development  of  the 
electroplaxes  is  to  be  found  in  the  end  of  the  tail  or  at  the  posterior  end  of 
the  spindle.  Thus  the  adult  structures  in  the  anterior  part  of  the  spindles 
represent  a  somewhat  inferior  and  less  complete  change  of  the  muscle  tissue 
into  electric  tissue  than  the  posterior  parts  of  the  same  organs  do. 

The  same  importance  attaches  to  the  fact  that  the  rates  of  development 
of  the  several  spindles  seem  to  vary.  The  lower  median  spindle  starts 
first  to  differentiate,  extends  farthest  forward  in  the  body,  is  larger  than  the 
others  when  developed,  and  during  early  development  is  always  in  advance 
of  the  corresponding  parts  of  the  other  spindles.  The  upper  median  spindle 
closely  follows  the  lower.  The  ventral  spindle  is  much  behind  the  two 
median  ones,  while  the  dorsal  spindle  represents  the  latest  and  weakest 
development  and  is  shorter  than  any  of  the  others.  These  facts  have  made 
it  possible  in  the  present  study  to  get  many  stages  of  development  from 
very  few  embryos. 

STUDIES  OF  AN   EMBRYO  NINE  DAYS  OF  AGE. 

This  little  fish  was  26  mm.  in  length  and,  while  the  egg-membrane  had 
been  ruptured  and  cast  away,  the  animal  was  still  forced  to  remain  in  its 
nest  because  of  its  huge,  elongate  yolk-sac,  still  unabsorbed,  and  because 
of  its  otherwise  undeveloped  organs  of  alimentation,  locomotion,  etc. 
The  posterior  part  of  the  body  was  carefully  cut  into  four  portions  (see 


i66  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 


text-fig.  5)  to  be  sectioned  as  follows:  First,  from  in  front  of  the  anus  to  a 
point  about  26  vertebral  segments  posteriorly.  The  last  three  segments 
of  this  portion  were  sectioned  transversely  and  serially  (region  A-^  while 
the  remaining  anterior  part  was  sectioned  vertically  and  longitudinally 
(region  A).  Another  portion  of  19  vertebral  segments  further  caudad  was 
removed  and  its  posterior  two  segments  sectioned  transversely  (regional), 
while  the  anterior  part  was  cut  as  before  in  vertical,  longitudinal  sections 


Fig.  5. — Outline  of  body  of  an  embryo  of  Gymnarchus  nine  days  old.  Transverse 
lines  and  letters  indicate  parts  sectioned  for  study.  For  explanations  see 
text.    (Copied  from  Assheton  in  "The  Work  of  John  Samuel  Budgett.")     X  S. 

(region  B).  A  third  part,  of  18  more  vertebral  segments,  was  treated  in 
the  same  way,  except  that  no  transverse  sections  were  taken  and  the  entire 
piece  was  cut  vertically  and  longitudinally  (region  C),  while  the  remaining 
portion  or  tail-tip  was  cut  serially  in  transverse  sections  and  forms  a  series 
(region  D). 

Figure  i,  plate  i,  shows  a  transverse  section  through  the  body  a  rather 
short  distance  from  the  extremity  of  the  tail  or  at  a  point  where  we  are  sure 
that  the  electric  organ  will  be  well  developed  a  little  later  in  life.  It  may 
be  thought  that  such  a  section  could  be  taken  for  study  to  better  advantage 
in  a  more  anterior  position  on  account  of  the  earlier  anterior  development 
just  discussed,  but  it  must  be  taken  into  account  that  the  tail  segments  are 
being  added  and  are  still  growing  rapidly  at  this  age,  that  they  are  very 
short  and  very  crowded,  and  therefore  the  location  of  this  section  is,  in 
reality,  fairly  well  forward  in  the  future  electric  spindles.  Conditions  were 
much  the  same  in  region  C. 

This  section  shows  a  good  development  of  muscle  fibers  as  indicated 
by  the  shaded  area.  As  is  usual  in  vertebrates,  the  most  advanced  stages 
of  muscle-cell  development  are  to  be  seen  at  the  lateral  periphery  of  the 
myotome.  Here,  at  the  point  indicated  in  figure  i ,  plate  i ,  by  the  dotted  line, 
a  layer  of  the  outer  muscle  cells,  two  or  three  deep,  has  acquired  an  average 
of  about  26  myofibrils  (an  average  of  14  counts).  These  are  grouped  in  one 
(fig.  2,  plate  I,  B  and  C)  or  sometimes  two  bundles  in  the  cell  (fig.  2,  plate  i, 
A)  and  their  correct  spacing  and  their  thickness  and  staining  power  indicate 
muscle  cells  of  normal  development  and  good  functional  activity.  The 
remaining  and  inner  muscle  cells  of  the  myotome,  forming  its  larger  bulk, 
show,  as  one  examines  them  successively  farther  inward  (toward  the  median 
line),  a  series  of  earlier  stages,  until  at  many  points  on  the  inner  edge  of  the 


Origin  of  Electric  Tissues  of  GyinnarcJms  Niloticus.  167 

myotome  the  smallest  cells  are  seen  with  large  nuclei  and  no  myofibrils  at  all 
(fig.  2,  plate  I,  C).  These  youngest  cells  are  particularly  abundant  at  the 
dorsal  and  ventral  edges  of  the  myotome. 

From  what  we  know  of  the  position  of  the  future  electroplaxes  and  their 
relation  to  the  muscle-masses,  we  can  be  sure  that  it  is  from  the  inner  edge 
of  the  myotome  that  the  electric  tissue  is  to  come,  and  a  close  scrutiny  of 
the  cells  which  form  this  edge  shows  that  at  two  points  only  is  there  any 
indication  of  such  a  development. 

One  of  these  points,  marked  with  a  circle  ( © )  in  figure  i,  plate  i,  is  v/here 
the  two  myotome  segments  (dorsal  and  ventral)  m^eet  and  close  in  against 
the  notochord.  Here  we  see,  in  some  of  the  sections,  several  especially 
large  and  strongly  developed  fibers,  somewhat  detached  from  the  rest  of 
the  myotome  and  resting  against  the  vertebral  disk.  Several  reasons  exist, 
however,  why  these  fibers  do  not  represent  the  future  electric  organ.  First, 
they  are  in  the  exact  median  position  which  remains  constant  during  growth 
and  in  which  no  electric  tissue  is  to  appear.  Second,  they  are  very  short 
and  are  not  attached  to  each  other  longitudinally,  as  other  fibers  are,  by 
means  of  connective  tissue,  but  are  attached  to  the  bodies  of  the  future 
vertebra.     They  may  be  called  the  vertebral  fibers. 

A  second  point  can  be  seen  where  some  muscle-tissue  shows  unusual 
development.  At  this  point  (marked  with  a  circle  (O)  in  fig.  i,  plate  i)  are 
several  muscle  cells  which  show  from  10  to  18  myofibrils  each,  and  a  degree 
of  development  almost  equal  to  the  cells  in  the  outer  layer.  These  cells 
are  represented  as  seen  under  high  magnification  in  figure  3,  plate  i.  They 
are,  I  believe,  destined  to  be  the  future  electric  cells,  several  of  which  will 
unite  and  form  one  of  the  electroplaxes  of  the  lower  median  spindle  of  the 
electric  organ.  I  base  this  assertion  only  on  their  position  and  their  some- 
what advanced  development  as  muscle  fibers,  for  they  show  at  this  time  no 
indication,  other  than  their  size,  of  developing  into  electric  tissue. 

Their  position  is  slightly  too  far  dorsal  for  the  lower  spindle,  in  an  adult 
Gymnarclms,  but  when  we  examine  the  12-day-old  and  42-day-old  stages, 
we  find  that  the  normal  growth  of  the  muscle-mass  will  carry  this  point  to 
exactly  the  proper  position  for  that  spindle  to  lie  at,  in  the  grown  fish. 

An  important  point  in  studying  these  cells  in  figure  3,  plate  i,  is  to  note 
that  they  are  separated  from  the  rest  of  the  myotome  and  from  one  another 
by  other  muscle  cells  of  weak  or  of  earlier  development  and  containing  only 
a  few  myofibrils.  These  weaker  cells,  and  even  some  of  the  more  advanced 
ones,  are  destined  to  degenerate  during  the  development  and  growth  of 
the  electroplax.  In  figure  3,  plate  i,  7  cells  are  present  that  will  probably 
take  part  in  the  formation  of  the  electric  spindle  at  this  point.  Some  of 
them  are  contiguous  and  others  are  widely  separated. 

In  figure  4,  plate  i,  we  have  a  fortunate  longitudinal  section  from  the 
adjacent  region  of  this  same  embryo.  The  section  shows  in  longitudinal 
view,  and  under  low  magnification,  the  same  group  of  muscle  cells  marked 
with  the  circle  (O)  at  both  ends.     Also,  it  shows  a  portion  of  the  ventral 


l58  Papers  from,  the  Marine  Biological  Laboratory  at  Tortugas. 

part  of  the  myotome  with  its  strongly  developed  outer  layer  {o.f.).  The 
vertebral  fibers  mentioned  above  are  not  visible,  but  their  position  in  a  few 
following  sections  is  indicated  by  a  line  one  can  imagine  to  be  drawn 
between  the  points  marked  with  the  figure  © .  It  has  been  considered 
unnecessary  to  figure  the  myofibrils  longitudinally  under  large  magni- 
fication. The  cross-striation  is  very  plainly  visible  and  is  the  same  in  all 
parts.  Each  of  several  regions  of  this  embryo  was  examined  and  in  all  its 
parts  were  the  same  conditions  found.  We  may  sum  up  by  stating  that  the 
embryo  of  9  days  age  shows  no  electric  tissue  and  but  a  very  weak  indication 
of  the  development  of  such  a  tissue,  all  of  the  myotome  cells  being  decidedly 
of  the  muscular  type.  Our  only  evidence  is  drawn  from  future  stages,  as 
to  the  position  of  such  electric  tissues  and  from  the  otherwise  unexplained 
precocity  of  the  fibers  in  our  9-day  embryo,  in  that  same  position. 


STUDIES  OF  AN   EMBRYO  TWELVE  DAYS  OF  AGE. 

This  animal  (text-fig.  6)  showed  a  considerable  increase  in  size,  not  so 
much  in  length  as  in  thickness.  The  posterior  part  of  the  body  of  this 
specimen  was  divided  into  the  following  pieces:  First,  from  a  short  dis- 
tance behind  the  anus  to  14 
vertebral  segments  further 
caudad.  Two-and-one-half 
segments  were  cut  off  of  this 
by  serial,  transverse  sections 
from  the  posterior  end  (re- 
gion ^  —  i)  and  the  remaind- 
er was  cut  into  vertical,  lon- 
gitudinal sections  from  right 
to  left  (region  A).  Second, 
15  segments  more  were  re- 
moved and  3  segments  of 
the  posterior  end  were  again 
cut  as  serial,  transverse  sections  (region  Bi)  and  the  remaining  12  segments 
were  cut  serially  in  horizontal,  longitudinal  sections  (region  B).  Third,  14 
segments  were  again  cut  off  and  the  posterior  3  segments  were  cut  trans- 
versely (region  Ci),  while  the  remainder  was  cut  in  vertical,  longitudinal  sec- 
tions (region  C).  The  next  18  vertebral  segments  formed  a  piece  that 
was  cut  in  vertical,  longitudinal  sections  (region  D),  without  any  transverse 
sections  being  made,  and  this  left  a  small  bit  of  somewhat  curled  tail-tip, 
which  was  so  young  in  development  that  its  segments  could  not  be  easily 
counted.     This  is  cut  transversely  in  series  (region  E). 

Study  of  this  embryo  may  best  begin  by  examining  a  transverse  section 
through  the  posterior  part  of  the  body  (fig.  5,  plate  2)  to  see  what  has 
become  of  the  inner  parts  of  the  myotomes.  Looking  first  for  the  point 
L.M.  as  seen  before  in  figure  i,  plate  i  (marked  with  O),  we  can  see  at  once 
that  it  is  present  as  a  compact  mass  of  muscle-like  tissue   now   clearly 


Fig.  6. — Diagram  of  body  of  an  embryo  of  Gymnarchtu  12  days 
old.  Lines  and  letters  indicate  the  regions  studied.  For  ex- 
planations see  text.  (Copied  from  the  same  source  as  fig.  5.) 
X  about  3.2s. 


Origin  of  Electric  Tissues  of  Gyninarchus  Nilotictts.  169 

separated  from  the  rest  of  the  myotome.  Further,  there  are  to  be  seen  on 
each  side  three  other  similar  sections  of  the  same  muscle-like  tissue,  all 
more  or  less  also  separated  from  the  main  mass  of  the  myotome.  The  most 
important  part  of  our  study  now  consists  on  the  one  hand  in  proving  that 
this  mass  L.M.,  in  figure  5,  plate  2,  is  derived  from  the  undoubted  muscle- 
fibers  (marked  with  O),  as  seen  in  figure  i,  plate  i,  and  on  the  other  hand  in 
showing  that  this  same  structure  is  to  become  the  finished  electric  tissue  as 
seen  in  such  advanced  development  as  in  figure  21,  plate  8,  for  instance. 

In  tracing  it  back  to  the  muscle,  we  are  much  assisted  by  the  fact  that 
the  dorsal  and  ventral  spindles  are  always  in  an  earlier  or  in  a  less  complete 
stage  of  development  than  the  upper  middle  spindle,  or,  particularly,  the 
lower  middle  spindle  under  consideration.  And,  since  in  this  embryo  of  12 
days  the  development  has  gone  to  considerable  length,  and  since  a  slightly 
younger  stage,  say  a  10-  or  11 -day  embryo,  was  not  included  among  the 
embryos  at  my  disposal,  this  fact  is  of  much  importance,  because  it  will  be 
fair  to  take  the  left  ventral  spindle  as  an  intermediate  step  in  the  compari- 
son. Figure  9,  plate  3,  is  a  highly  magnified  section  of  the  locality  of  the 
left  ventral  spindle  from  region  C  in  the  embryo  of  12  days,  and  in  it  we  can 
see  a  mass  of  muscle-like  cells,  closely  associated  and  lying  at  the  inner 
edge  of  the  myotome.  Certain  changes  clearly  differentiate  them  from  the 
rest  of  the  muscle,  however.  One  change  is  the  fact  that  the  myofibrils 
have  shown  a  large  diminution  in  size  or  thickness  and  have  also  suffered  in 
power  to  take  the  stain ;  particularly  on  the  periphery  of  some  of  the  muscle- 
columns  they  almost  refuse  to  take  it.  Their  spacing  is  also  irregular  and 
they  show  a  distinct  tendency  to  clumping  together  and,  in  some  cells,  to 
get  close  to  the  nucleus  or  even  to  surround  it.  They  can  be  readily  com- 
pared in  figure  9,  plate  3,  with  the  well-developed  young  muscle  cells  just 
outside  and  to  the  left  of  them.  The  dotted  line  marked  XX  indicates  a 
separation  of  the  two.  All  of  those  to  the  right  of  this  line  show  the  con- 
dition of  the  myofibrils  mentioned  above;  those  to  the  left  show  the  usual 
condition  of  muscle  cells  of  this  age  in  fishes. 

A  second  characteristic  of  the  changing  muscle  cells  under  discussion 
is  in  their  cytoplasm.  It  appears  more  abundant,  although  this  may  be 
due  to  the  smaller  fibril  bundles.  But  it  also  stains  more  heavily  with 
such  stains  as  eosin,  erythrosin,  and  orange  G.  With  the  eosin,  for  example, 
it  also  shows  a  more  yellowish  tinge  than  the  cytoplasm  of  the  usual  muscle 
cells  in  the  same  sections.  And  lastly,  some  of  the  muscle  cells  seem  to 
have  entirely  disappeared  or  to  have  greatly  shrunken.  This  latter  fact 
causes  a  loose  and  separated  condition  to  obtain  among  the  metamorphosing 
cells  which  shows  in  sharp  contrast  (fig.  9,  plate  3)  to  the  compact  con- 
dition seen  in  the  typical  muscle  cells  to  the  left  of  or  outside  of  the  line  XX. 

Another  important  fact  can  be  seen  among  the  changing  cells  in  figure 
9.  Those  near  the  center  of  the  group  show  a  tendency  to  touch  or 
coalesce  with  each  other.  Already  at  this  early  stage  this  marks  a  differ- 
ence in  the  group.     Those  cells  within  the  dotted  circle  are  destined  to 


I/O  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

unite  with  one  another  in  a  compact  bundle  to  form  the  future  electroplax; 
while  those  of  the  group  which  lie  outside  of  the  dotted  ring  are  to  degenerate 
and  atrophy  altogether,  in  order  to  make  room  for  the  growing  electroplax. 
Just  mediad  of,  or  to  the  right  of,  the  dotted  ring  is  seen  a  cell  that  has 
entirely  lost  its  myofibrils  and  whose  cytoplasm  is  vacuolated  and  about  to 
be  absorbed.  At  other  points,  to  the  left,  still  other  cells  can  be  seen  that 
have  dwindled  to  a  smaller  size  than  any  of  the  normal  muscle  cells.  The 
cells  within  the  ring  are  judged  by  the  writer  to  be  those  which  will  form 
the  electroplax  at  this  point,  because  they  are  starting  to  unite  and,  also, 
because  they  occupy  the  position  at  which  the  electroplax  will  lie.  Another 
reason  is  that  their  myofibrils  are  weakening  and  clumping.  Some  of  those 
cells  lying  inside  of  the  ring  may  also  atrophy.  This  can  not  be  infallibly 
judged;  but  certainly  all  of  those  outside  of  it  and  to  the  right  of  the  line 
XX  are  about  to  degenerate  and  are  not  found  in  later  stages. 

The  connective  tissue  creeps  into  the  neighborhood  of  and  among  these 
metamorphosing  muscle  cells  at  this  time,  and  good  mitotic  figures  can  be 
seen,  showing  that  it  is  increasing  the  number  of  its  cells.  It  does  not, 
however,  penetrate  the  groups  of  future  electric  cells  inside  the  dotted  line. 
Also  blood,  pigment,  etc.,  are  to  be  seen  in  characteristic  positions. 

A  transverse  section  of  the  body  at  region  Bi  need  not  be  illustrated  at 
this  point  by  a  low  magnification  figure,  because  it  is  so  like  figure  5,  plate  2, 
in  general  appearance.  But  it  happens  that  in  such  a  section  several  very 
interesting  stages,  forming  a  sequence  of  which  figure  9,  plate  3,  can  be  taken 
as  the  first  member,  were  noticed,  and  figure  10,  plate  3,  is  the  second  in 
this  series.  This  drawing  represents  the  right  ventral  spindle  in  region  Bi, 
and  a  number,  five  to  be  exact,  of  the  transforming  cells  can  be  seen  here  in 
closer  union  than  the  corresponding  cells  were  in  figure  9,  plate  3.  One  or 
possibly  two  of  these  may  disintegrate  a  little  later.  A  dotted  ring  is  not 
necessary,  because  the  connective-tissue  cells  have  partly  marked  off  the 
electric  cells,  and  outside  of  this  incomplete  ring  and  above  it  can  be  seen 
two  muscle  cells  which  are  atrophying.  A  third  cell  is  shown  in  a  final 
stage  of  disintegration.  Its  cytoplasm  is  almost  clear,  or  all  gone,  and  its 
myofibrils  have  united  in  a  single  lump,  which  will  soon  become  a  round 
droplet  or  cell-inclusion  that  will  afterwards  disappear. 

Moving  to  the  right  upper  middle  spindle  (fig.  1 1 ,  plate  3) ,  we  need  very 
little  explanation  to  see  how  the  four  or  more  muscle  cells  that  first  com- 
posed this  structure  have  come  into  a  still  closer  union.  Below  and  to  the 
right  (outer)  are  still  seen  some  of  the  degenerating  muscle  cells — five  in 
this  figure. 

■  Several  important  points  must  be  discussed  in  connection  with  this 
figure;  the  component  cells,  as  seen  here,  are  not  simple,  single  muscle 
fibers.  The  lower  one  can  easily  be  seen  to  have  two  myofibril  bundles  as 
well  as  two  nuclei.  The  presence  of  two  widely  separated  fibril  bundles, 
as  well  as  the  large  size  of  the  cell,  makes  it  nearly  certain  that  the  structure 
was  formed  by  the  coalescence  of  two  muscle  fibers.     In  the  upper  region 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus.  171 

of  the  future  electroplax,  however,  a  large  cell  is  seen  which  has  only  one 
fibril  bundle  and  four  nuclei.  It  is  possible  here  that  this  was  one  cell  and 
that  it  is  growing  in  size  and  multiplying  its  nuclei  by  amitotic  division. 
As  this  is  the  same  process  which  goes  on  in  ordinary  muscle  cells  of  this 
age,  it  is  not  surprising  to  find  it  going  on  here,  and  in  older  specimens  we 
shall  find  it  the  rule.  The  cytoplasm  of  all  these  electric  cells  is  abundant 
at  this  stage  and  is  dense  staining  with  the  acid  dyes. 

As  compared  with  figure  11,  plate  3,  the  next  illubtration,  figure  7,  plate 
2,  is  most  interesting  and  is  a  step  of  some  magnitude  in  the  development 
of  the  electroplax.  The  cytoplasm  of  such  cells  as  compose  this  young 
electroplax  is  all  united  into  a  single  mass  and  the  relation  of  nuclei  to  fibril 
bundles  is  completed.  The  nuclei  are  always  peripheral  and  the  fibril  bun- 
dles appear  to  form  a  single  central  mass.  This  is  the  permanent  condition 
which  will  obtain  throughout  the  life  of  the  organ,  and  is  also  the  condition 
common  to  some  other  electroplaxes,  as,  for  instance.  Raja  and  Mormyrus. 

Just  how  the  several  fibril  bundles  become  massed  as  a  single  bundle  is 
not  to  be  positively  stated  at  this  time.  The  individual  bundles  can  hardly 
be  imagined  as  moving  together  through  the  cytoplasm.  It  is  probable 
that  some  of  the  several  bundles  as  seen  in  figure  11,  plate  3,  are  lost  and 
absorbed,  but  it  is  not  probable  that  all  but  one  are  so  removed.  The  central 
bundle  in  figure  7,  plate  2,  looks  large  enough  to  be  composed  of  several, 
such  as  are  seen  in  figure  11,  plate  3.  The  best  explanation  is  that  several 
remaining  bundles  are  moved  toward  each  other  by  growth  currents  in  the 
cytoplasm,  or  by  the  absorption  of  material  which  lies  between  them.  At 
the  same  time,  of  course,  the  peripheral  cytoplasm  is  growing  in  mass  and 
all  nuclei  tend  to  remain  in  this  external  layer.  We  shall  see  later  that  a 
very  few  nuclei  are  left  behind  in  the  central  fibrous  mass  in  some  electro- 
plaxes. 

In  figure  7,  plate  2,  we  see,  plainly  and  indubitably,  the  first  form  of 
the  electroplax  as  found  in  older  fishes. 

The  connective  tissues  which  surround  the  electroplax  are  becoming 
more  decided  in  figure  7,  plate  2.  Also  blood-vessels  and  pigment-cells  are 
oftener  seen  as  in  this  drawing.  One  muscle  cell,  with  its  myofibrils  clotted 
into  several  irregular  masses  and  its  nucleus  in  an  advanced  stage  of  dis- 
integration, is  seen  just  between  the  lower  end  of  the  pigment-cell  and  the 
electroplax. 

It  will  be  well  at  this  point  to  examine  some  of  the  longitudinal  sections 
of  these  early  stages,  in  order  to  make  clear  several  points  which  can  not 
be  so  well  studied  in  the  transverse  views. 

Figure  12,  plate  4,  is  a  low  magnification  picture  (X  140)  of  6  segments 
of  the  caudal  part  of  the  body  at  region  D  in  this  12-day  embryo.  Only 
the  dorsal  part  of  the  body  is  shown,  where  a  fortunate  slant  of  the  section 
has  permitted  the  knife  to  pass  through  both  the  dorsal  and  the  upper 
median  spindles  at  the  same  time.  The  drawing  is  an  accurate  projection 
from  three  different  sections,  so  that  all  parts  of  each  spindle  might  be 


172  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

shown,  as  well  as  their  relations  to  the  rest  of  the  myotomes.  Bracket 
No.  I  (zone  i)  in  this  figure  indicates  the  inner  tips  of  the  ventral  halves  of 
the  myotomes.  Zone  2  shows  a  mass  of  connective  tissue  and  nerve  which 
runs  between  these  two  halves  (compare  fig.  5,  plate  2,  Conn  T.).  Zone  3 
embraces  the  inner  tips  of  the  dorsal  myotomes  where  they  lie  between 
the  upper  median  spindle  and  the  horizontal  connective-tissue  septum. 
It  will  be  noticed  that  this  set  of  muscle  fibers  appears  to  be  very  short, 
much  shorter  than  the  connective-tissue  regions  between  them  (not  filled 
out  in  this  drawing).  The  reason  can  be  seen  at  once  when  these  same 
muscle  cells  are  examined  under  the  high  power  (fig.  15,  plate  5,  bracket  3; 
and  fig.  6,  plate  2),  where  it  is  apparent  that  they  are  degenerating  fibers. 
Parts  of  four  of  these  fibers  are  shown  in  figure  15,  plate  5,  and  that  one 
nearest  the  electric  tissue  which  is  represented  in  figure  15,  plate  5  (brackets 
4  and  4i),  is  the  farthest  gone,  having  lost  its  myofibrils  altogether.  The 
nuclei  are  also  distorted  and,  besides  the  single  large  nucleolus,  are  noticeably 
empty  of  any  chromatic  matter  or  even  of  linin  (notice  the  nuclei  of  healthy 
muscle  and  electric  tissue).  The  next  fibers  to  the  left  in  this  figure  serve 
well  to  show  how  the  degeneration  takes  place  from  the  ends,  where  the 
cytoplasm  is  gathered  in  heavy  lumps  which  stain  light  and  yellowish  with 
eosin.  In  fact,  there  is  a  singular  similarity  between  the  process  of  degen- 
eration of  these  muscle  cells  and  the  transformation  of  the  others  into  the 
electric  tissue.  Above,  in  zone  3,  is  a  rounded  cell  which  I  take  to  be  a 
muscle  cell  in  a  final  stage  of  dissolution.  It  contains  large  granules  of  a 
chromatic  substance,  which  appear  to  be  the  remains  of  myofibrils.  Figure 
6,  plate  2,  also  represents  three  fibers  from  this  same  region  in  another 
myotome  and  shows  three  stages  of  the  degeneration  of  the  muscle.  Two 
of  them  indicate  that  the  degeneration  begins  in  the  middle  of  the  fiber. 

Zones  4  and  5  in  figure  15,  plate  5,  as  well  as  in  figure  12,  plate  4,  show  the 
young  electric  tissue  of  the  upper  median  spindle.  Let  us  first  consider  this 
tissue  in  figure  12,  plate  4,  where  we  can  get  a  larger,  low-powered  view  of 
it.  In  the  first  place,  the  most  noticeable  feature  of  the  electric  tissue  here 
is  that  it  shows  scarcely  any  signs  of  a  division  into  the  original  myotomes. 
A  trace  of  this  is  seen  in  zone  4,  but  in  a  careful  and  systematic  search 
through  a  number  of  the  spindles  of  this  stage  very  few  instances  could  be 
noted.  Even  the  myofibril  bundles  of  successive  myotomes  seem  to  have 
united,  and  these  bundles  were  most  carefully  examined  under  a  Zeiss  2  mm. 
1.40  ap.  lens.  I  have  no  doubt  that  each  spindle  does  constitute  a  con- 
tinuous reticulum  of  muscle  cells  at  this  period.  Laterally,  the  various 
muscle  cells  are  not  continuously  united,  but,  as  is  shown  in  figure  15,  plate  5, 
they  are  united  at  a  number  of  points  much  in  the  way  that  some  heart- 
muscle  fibers  are. 

The  myofibrils  are  best  seen,  of  course,  in  these  longitudinal  sections, 
and  it  can  be  seen  that  they  are  typical.  The  transverse  stria tion  is  as 
perfect  as  in  the  functional  muscle,  but  owing  to  the  slighter  fibrils  they  are 
not  quite  so  dark.     In  the  upper  median  spindle  of  figure  15,  plate  5,  some 


Origin  of  Electric  Tissues  of  Gymnarchiis  Niloticus.  173 

points  were  found  where  this  striation  was  slightly  weakened.  Cases  of 
relaxation  and  semi-contraction  uniformly  characteristic  of  the  normal 
muscle  prevailed.  The  distances  between  the  bands,  as  well  as  the  length 
of  the  anisotropic  parts,  was  the  same  as  in  ordinary  muscle  and  remains 
as  long  as  striation  can  be  seen.  This  same  fact  does  not  hold  true  in  Raja 
or  in  Astroscopus,  where  it  is  much  shortened.  It  does  hold,  however, 
in  Mormyrus,  which  is  closely  related  to  Gymnarchus. 

The  longitudinal  sections,  shown  in  figure  12,  plate  4,  and  figure  15,  plate 
5,  correspond  to  the  spindles  before  it  has  become  possible  to  see  that  they 
have  segmentally  divided  into  electroplaxes  and  before  a  central  core  of 
fibrous  material  has  become  definitely  differentiated  from  a  superficial  layer 
of  cytoplasm  containing  all  of,  or  nearly  all  of,  the  nuclei. 

We  will  now  advance  toward  the  head  of  this  same  specimen,  to  find 
material  in  a  more  advanced  stage  of  development,  in  order  to  study  the 
segmentation  of  the  embryonic  spindle  into  its  individual  electroplaxes 
(this  segmentation  has  just  been  described  as  absent  in  fig.  12,  plate  4), 
and  to  study  further  the  differentiation  of  the  inner  fibrous  core  from  the 
outer  layer,  and  lastly  to  see  how  the  myofibrils  lose  their  transverse  stri- 
ation (muscle  striation).  The  reader  will  remember  that  at  this  stage  the 
anterior  electroplaxes  are  in  advance,  developmentally,  of  the  posterior  ones. 
Figure  13,  plate  4,  shows  four  vertebral  segments,  taken  from  the  region  C 
of  this  same  embryo,  under  low  magnification.  Bracket  i  embraces  the 
zone  of  the  epithelium.  No  connective  tissue  is  shown.  Between  the 
muscle-zones  2  and  4  lies  the  zone  of  the  long,  narrow,  embryonic  electric 
spindle,  marked  by  bracket  3.  It  is  the  lower  median  spindle  which  is 
shown,  one  which  is  always  in  advance  of  all  the  others  in  development. 
In  this  figure  it  becomes  quite  plain  that,  while  the  muscle  tissue  is  arranged 
segmentally  to  correspond  to  the  vertebra,  the  electric  organ  is  not.  In 
this  case  the  electric  spindle  is  separated  transversely  by  connective  tissue 
into  three  segments,  instead  of  four.  Nor  is  this  proportion  always  main- 
tained among  the  several  electric  spindles  themselves. 

In  some  cases,  as  will  be  seen  later,  a  single  electroplax  corresponds  to 
as  many  as  five  vertebrae.  Even  the  electroplaxes  do  not  correspond  with 
each  other.  A  subsequent  figure  will  also  demonstrate  this  (fig.  23,  plate  9). 
Just  what  factors  do  determine  the  length  of  the  electroplax  I  am  unable 
to  say.  A  closer  study  of  the  nerve  distribution  and  blood-supply  may 
throw  some  light  on  the  matter. 

The  transverse  sections  of  the  stages  represented  in  figure  13,  plate  4,  do 
not  show  anything  of  interest  over  and  above  what  was  discussed  in  con- 
nection with  the  conditions  seen  in  figures  9,  10,  and  11,  plate  3,  and  figure  7, 
plate  2.  We  will,  therefore,  advance  toward  the  head  one  step  further  in 
this  embryo  and  examine  a  longitudinal  section  from  the  region  marked  B 
in  text-figure  6.  Here  (fig.  17,  plate  6)  the  electric  tissue  is  seen  in  what  may 
be  considered  as  its  maximum  development  in  this  12-day  embryo.  The 
electroplaxes  are  distinct  from  each  other  and  have  grown  considerably  in 


174  Papers  from  the  Marine  Biological  Laboratoryat  Tortugas. 

size.  The  one  selected  for  illustration  is  from  a  lower  median  spindle  and 
shows  an  actual  length  of  0.9  mm.  It  appears  with  smooth,  well-defined 
boundaries  and  good  separation  of  core  and  outer  layer.  The  myofibrils 
are  shown  in  the  core  as  several  closely  associated  bundles,  only  to  be 
distinguished  from  one  another  by  a  slight  curving  and  twisting  in  their 
course.  This  would  not  be  visible  in  a  transverse  section  at  most  levels  in 
the  electroplax,  and  it  gives  strength  to  the  view,  expressed  before,  that  the 
fibril  bundles  of  a  number  of  young  muscle  cells  combine  to  form  the  single 
fibrous  core  of  each  electroplax.  At  the  levels  examined  it  would  appear 
that  a  group  of  from  4  to  7  cells  from  each  myotomie  is  concerned  in  the 
formation  of  any  electroplax  and  that  these  same  groups  of  from  2  or  3 
myotomes  are  likewise  united  end  to  end,  thus  making  it  possible  that 
from  12  to  21  muscle  cells  are  united  to  form  each  electroplax. 

The  beginning  of  the  loss  of  transverse  striation  is  quite  visible  in  figure 
17,  plate  6.  This  striation  merely  fades  or  loses  its  staining  power,  beginning 
at  several  points,  but  usually  at  the  middle  of  the  electroplax.  The  ends 
retain  the  staining  power  of  the  M  stripes  longest. 

The  multiplication  of  nuclei  mmst  be  spoken  of  at  this  point.  It  is 
well  known  that  the  nuclei  of  future  muscle  tissue  divide  by  mitosis  with  a 
consequent  and  subsequent  division  of  the  cell-body  as  long  as  the  cells  are 
in  the  young  myoblast  stage.  As  soon  as  muscle  differentiation  begins  to 
take  place,  the  mitotic  division  of  nuclei  is  changed  to  an  amitotic  type  of 
division  and  the  cell-body  ceases  to  divide  and  begins  to  lay  down  myo- 
fibrils. Some  few  observations  against  this  view  are  recorded,  but  it  seems 
to  hold  in  Gymnarclius. 

In  figure  15,  plate  5,  one  can  readily  see  many  cases  of  amitotic  nucleus 
division.  The  few  cases  of  mitosis  visible  in  the  figure  are  of  connective- 
tissue  cells.  Such  cells,  it  is  known,  always  divide  by  mitosis  during  their 
whole  existence.  As  the  electroplax  grows,  more  nuclei  are  needed,  and  they 
are  supplied  by  the  amitotic  divisions  above  mentioned.  Just  how  long  this 
process  keeps  up  is  not  known,  but  it  is  probably  mostly  done  before  the 
first  15  or  20  days  of  development  are  passed.  It  is  evidently  going  on 
fast  in  the  12-day  embryo,  and  it  is  possibly  finished  in  the  42-day  stage. 
The  nuclei  are  typical  muscle  nuclei  in  appearance  and  are  not  to  be  dis- 
tinguished from  the  nuclei  of  real  and  active  muscle  in  the  same  preparations, 
except,  perhaps,  they  are  slightly  larger  and  have  a  somewhat  heavier  chro- 
matic content  (larger  nucleolus  and  chromatic  granules).  This  does  not 
hold,  of  course,  for  the  degenerating  nuclei,  whose  differences  have  already 
been  described.  Connective-tissue  nuclei  can  at  once  be  distinguished  by 
their  delicate  outline  and  small  chromatin  content,  which  is  distributed  as 
very  small  granules. 

STUDIES  OF  A  LARVA  FORTY-TWO  DAYS    OLD. 
In  order  to  continue  tracing  this  history  of  the  development  and  growth 
of  the  electric  tissue,  we  will  now  be  obliged  to  pass  to  an  embryo  of  some 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticits.  175 

considerable  size  and  one  in  which  the  oldest  tissues  are  almost  the  same, 
for  an  understanding  of  the  adult  structure  as  those  of  a  fully  grown  fish 
would  be.  Text-figure  7  gives  an  outline  of  this  specimen,  which  was  about 
63  mm.  long  and  whose  tail  part  was  cut  off  and  divided  and  sectioned  as 
follows:  Beginning  at  about  the  level  of  the  anus,  a  portion  composed  of  6 
vertebral  segments  was  sectioned  in  a  vertical  and  longitudinal  direction 
(region  A) ;  then  passing  caudad,  the  next  2  segments  were  cut  transversely 
(region  Ai);  then  the  next  12  were  cut  longitudinally  and  horizontally 
(region  C) ;  then  the  next  6  were  cut  transversely  (region  Ci) ;  then  13  others 
were  cut  longitudinally  and  vertically  (region  D) ;  then  7  more  were  cut 
transversely  (region  Di);  then  the  next  14  were  cut  longitudinally  and 
approximately  vertically  (region  E),  while  the  tip  of  the  tail,  composed  of 
some  12  or  more  segments,  was  sectioned  transversely  (region  £1). 

In  the  embryo  of  12  days  the  youngest  developmental  stages  of  the 
electric  tissue  were  found  in  the  most  distal  portion  of  the  tail,  which  at  that 
time  had  but  recently  been  extended  by  growth  from  the  body  and  was  still 
in  process  of  extension.  The  oldest  and  consequently  the  most-developed 
electroplaxes  were  to  be  found  in  the  anterior  or  cephalic  end  of  the  spindles. 

In  the  present  embryo,  or  larva,  of  42  days,  the  conditions  are  reversed, 
and  the  electroplaxes  in  the  extremity  of  the  tail  have  passed  those  farther 
up  in  the  body  in  their  differentiation,  and  have  reached  a  much  greater 
and  more  complete  development.  Accordingly  we  will  select  for  study  one 
of  the  most  anterior  and  least  developed  examples  and  compare  it  with  that 
electroplax  last  studied,  which  is  represented  by  figure  17,  plate  6. 

Figure  18,  plate  6,  is  drawn  from  one  of  the  ventral  electroplaxes  of  the 
region  C,  as  shown  in  text-figure  7.  This  position  makes  it  fairly  well  forward 
in  the  spindle,  although  portion  B  v/ould  have  shown  slightly  younger  stages. 


Fig.  7. — Diagram  of  body  of  an  embryo,  or  larva,  of  Gymnarclms,  about  42  days  old. 

Lines  and  letters  indicate  regions  studied.     For  explanations  see  text. 
(Copied  from  the  same  source  as  text-fig.  5.)      X  about  i  and  1.5. 

The  fibril  core  will  first  attract  our  attention.  The  first  noticeable 
feature  is  that  this  core  is  growing  in  mass  and  volume  all  through  the 
electroplax,  but  also  far  faster  in  its  center  than  at  either  end.  Throughout 
its  course  it  has  assumed  a  unified  appearance  which  shows  no  trace  of  the 
several  fibril  bundles  which  have  gone  to  compose  it.  In  the  narrower  ends 
the  mass  is  straightest  and  its  component  fibers  appear  most  parallel,  being 
but  slightly  wavy.  As  we  follow  them  from  either  end  towards  the  middle 
it  can  be  seen  that  their  course  becomes  more  and  more  wave-like,  until, 
in  the  middle,  they  have  been  thrown  into  decided  folds.     Their  actual 


176  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

course  can  be  somewhat  better  followed  with  thicker  sections  and  deep 
eosin  staining  than  with  preparations  of  the  usual  kind. 

As  to  the  fibrils  themselves,  they  have  lost  the  transverse  striation 
altogether.  By  this  I  mean  that  it  is  no  longer  stainable.  It  seems  that, 
in  the  straight  ends,  with  a  good  immersion  lens  one  can  see  traces  of  this 
striation,  left  as  a  thickening  of  the  fibrils,  at  points  that  may  represent 
the  previous  position  of  the  anisotropic  M  spindles. 

Whether  the  increase  in  the  mass  of  the  fibrillar  core,  which  goes  on  as 
the  electroplax  grows,  is  due  to  the  thickening  of  the  individual  fibrils  or  to 
the  laying  down  of  more  and  new  fibrils  or  to  the  deposit  of  an  interfibrillar 
substance  between  them,  was  not  decided.  The  first  two  conditions  seemed 
the  most  probable,  because  of  the  apparent  absence  of  much  interfibrillar 
substance  in  the  oldest  and  largest  electroplaxes. 

The  cytoplasm  of  the  outer  layer  begins  to  be  of  interest  at  this  stage. 
Its  most  particular  point  of  interest  lies  in  the  fact  that  it  is  decidedly 
different  in  structure  at  its  anterior  end  from  its  posterior  end.  This  is 
shown  in  its  staining  capacity  as  well  as  in  its  actual  structure.  In  the 
specimens  stained  in  iron  haematoxylin  and  eosin  the  cytoplasm  at  the 
posterior  end  stains  deeper  than  that  at  the  anterior  end,  with  both  dyes. 
It  can  also  be  seen  to  be  granular  in  structure — a  sort  of  general  granulation 
with  a  few  larger  granules  of  a  substance  which  stains  somewhat  like  chro- 
matin. In  particular,  its  cytoplasm  is  darker  than  the  fibrillar  core  at  this 
posterior  end. 

As  the  cytoplasmic  layer  is  examined  in  an  anterior  direction,  it  is  seen 
to  stain  lighter  and  at  the  same  time  to  contain  an  occasional  vacuole.  This 
condition  increases  until,  at  the  anterior  end,  about  one-fifth  of  the  length 
of  the  entire  structure  is  covered  with  a  cytoplasm  which  is  so  much  vacuo- 
lated that  it  appears  as  a  delicate  reticulum  in  the  meshes  of  which  the 
nuclei  lie.  The  fibril  core  extends  out  to  the  end  or  almost  to  the  end,  and 
it  can  be  seen  that  it  is  darker  in  color  and  denser  than  the  cytoplasm 
covering  it.  Some  of  the  finer,  granular  material  is  found  out  as  far  as  the 
reticulated  tip. 

A  new  element  of  interest  begins  to  become  apparent  in  this  stage,  and 
that  is  the  point  of  attachment  of  the  nerve.  The  strong  medullated 
fibers  come  from  the  cord,  pass  through  the  spinal  ganglion,  and  enter  the 
connective-tissue  "tube"  of  the  spindle  at  the  level  of  the  posterior  third  of 
each  electroplax.  The  fibers  wind  and  turn  considerably,  in  this  vicinity, 
and  are  finally  applied  to  the  surface  of  the  electroplax  over  an  area  which 
may  be  described  in  this  specimen  as  its  second  sixth  part  from  the  posterior 
end.  Thus  the  extreme  posterior  end  does  not  receive  any  nerve-endings 
at  this  period. 

The  cytoplasm  shows  a  number  of  indentations  where  the  nerve  is  ap- 
plied and  the  axis-cylinders  of  the  nerve  can  be  traced  into  these  spaces, 
which  they  apparently  fill  with  a  club-shaped  nerve-ending.  This  ending 
can  not  be  satisfactorily  described  until  some  of  the  special  nerve  methods 
can  be  used  to  elucidate  it. 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloiicus.  177 

Another  and  more  simple  step  in  development  is  indicated  in  figure  19, 
plate  6,  which  was  taken  from  an  electroplax  in  a  dorsal  spindle  of  this 
embryo  at  the  region  E  (see  text-fig.  7) . 

Two  points  of  interest  will  be  spoken  of  in  connection  with  this  stage. 
The  shape  has  changed  as  follows:  The  middle  part  has  both  actually  and 
comparatively  widened  over  the  breadth  shown  by  the  middle  part  of  the 
electroplax  seen  in  figure  18,  plate  6,  and  this  increased  width  is  due  to  a 
broadening  of  the  fibrous  core  alone,  the  outer  nucleated  layer  remaining 
the  same. 

Accompanying  this  widening  is  also  an  actual  shortening  of  the  structure. 
Thus,  part  of  the  increased  bulk  of  the  middle  is  due  to  an  absorption  of  the 
two  ends.  Apparently  the  anterior  end  suffers  the  greater  amount  of 
absorption,  for  it  is  usually  shorter  and  thinner  than  the  posterior.  In 
this  connection  we  must  remember  the  previous  vacuolization  of  this 
anterior  end  as  seen  in  figL.re  18,  plate  6.  It  would  appear  that  the  vacuoli- 
zation was  part  of  an  absorption,  or  rather  of  an  atrophic  process.  Con- 
siderable traces  of  it  remain  in  figure  19,  plate  6,  and  it  is  also  still  noticeable 
that  the  anterior  end  does  not  stain  deeply.  The  posterior  end  retains  its 
length  and  vigor  to  a  greater  degree.  It  seems  to  become  considerably 
narrowed,  however. 

The  form  of  the  electroplax  also  begins  to  show  a  marked  change  through 
the  development  of  spurs,  branches,  or  papillje  which  begin  to  protrude  from 
both  its  ends,  particularly  from  its  posterior  end.  Some  indication  of  this 
was  visible  already  in  figure  18,  plate  6,  as  small  lumps  or  "shoulders"  that 
marked  the  organ  roughly  into  three  parts — anterior,  middle,  and  posterior 
thirds.  These  growing  spurs  begin  to  give  the  middle  third  a  distinct 
truncate  or  cylindrical  form.  The  nerve  fibers  also  attach  themselves  to 
the  sides  of  the  growing  papillae,  particularly  to  their  bases.  The  papillae 
show  an  inclination  to  grow  out  of  the  sides  of  the  posterior  third. 

We  will  now  pass  to  the  last  and  oldest  stage  of  the  electroplax  which 
can  be  found  in  the  embryos  which  Budgett  collected  in  Africa.  This  is 
found  in  the  E  region  of  the  42-day  embryo  or  larva  of  Gymnarchus  and  is 
represented  in  longitudinal  sections  under  a  low  magnification  by  figure  23, 
plate  9,  which  shows  parts,  or  the  whole,  of  about  11  electroplaxes  lying  in 
place  in  the  ventral,  lower  middle,  and  upper  middle  electric  spindles  of 
this  region.  Figure  21,  plate  8,  also  shows  a  transverse  section  just  cephalad 
of  this  point,  in  the  D  region,  where  cross-sections  of  the  8  electric  spindles 
reveal  partial  or  complete  transverse  sections  of  7  electroplaxes,  but  only 
the  electric  connective  tissue  which  fills  the  tube  at  this  point,  in  the  eighth. 
Figure  20,  plate  7,  represents  a  longitudinal  section  of  one  of  the  electro- 
plaxes shown  in  figure  23,  plate  9,  and  will  serve  as  a  basis  for  our  last  study 
of  this  series. 

The  form  has  continued  to  follow  the  development  which  was  indicated  in 
figure  19,  plate  6.  The  total  length  of  the  structure  has  shortened  somewhat 
more,  while  its  width  in  the  middle  part  has  increased  twofold.     These 


178  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

statements  are  the  result  of  averages  of  about  15  measurements.  The 
papillse  on  the  posterior  surface  have  increased  in  length  and  some  of  them 
have  begun  to  rival  the  posterior  portion  of  the  organ  in  length.  We  have 
no  suitable  figures  of  the  adult  electroplax,  but  from  Fritsch's  (3)  descrip- 
tions, and  from  the  tendencies  shown  by  these  embryos,  it  would  seem  that, 
as  Ewart  describes  in  Raja,  the  original  posterior  portion  of  the  electroplax 
shortens  and  the  new  papillae  lengthen  until  they  all  form  approximately 
similar  structures.  This  can  only  be  completely  studied  when  we  have 
secured  suitable  sections  of  the  grown  fish. 

The  development  of  papillae  is  noticeably  weak  on  the  anterior  surface. 
The  figure  does  not  show  as  many  as  some  of  the  electroplaxes  in  figure  23, 
plate  9,  but  it  is  a  fair  illustration.  Neither  does  it  show  well  the  usual 
condition  of  the  main  anterior  process  of  the  cell  at  this  time,  which  can  be 
better  seen,  in  some  electroplaxes  of  figure  23,  plate  9,  to  be  still  in  evidence 
and  of  considerable  length,  but  of  very  weak  development.  This  anterior 
process  shows  no  trace  at  this  age  of  the  general  vacuolization  of  its  cyto- 
plasm which  we  saw  in  an  earlier  stage,  and  the  fibrils  extend  as  a  very  thin 
and  uncertain  core  through  its  length. 

The  fibrillar  mass  which  forms  the  core  of  the  electroplax  has  now 
assumed  what  appears  to  be  its  permament  condition.  The  fibrils  are 
very  fine  and,  after  having  passed  straight  down  through  the  posterior 
process,  are  thrown  into  flat  waves  by  the  process  of  packing  them  into 
the  shortening  and  widening  middle  part  which  now  constitutes  the  principal 
bulk  of  the  structure.  This  causes  the  larger  part  of  the  fibrils  to  lie  nearly 
at  right  angles  to  the  longitudinal  axis  of  the  organ,  and  this  appearance 
was  first  taken  by  the  writer,  who  examined  the  oldest  embiyo  first,  to  be  a 
trace  of  the  transverse  striation  of  muscle  from  which  the  organ  is  formed. 
We  now  know  it  to  be  the  myofibrils,  lying  at  right  angles  to  their  original 
course.  No  conclusion  was  arrived  at,  in  this  stage,  concerning  the  growth 
of  this  mass,  as  to  whether  the  number  of  fibrils  was  increased,  or  whether 
the  larger  size  was  due  to  a  thickening  of  the  original  fibers,  or  to  the  deposit 
of  interfibrillar  substance.  The  fibrils  seemed  to  be  as  fine  if  not  finer  than 
in  the  earlier  stages ;  certainly  they  are  much  finer  than  functional  myofibrils. 
Since  muscle  tissue  increases  the  number  of  its  myofibrils  long  past  this  age, 
I  see  but  little  reason  why  this  modified  muscle  should  not  also  do  so. 

A  closer  study  of  the  cytoplasm  and  nuclei  of  the  peripheral  layer  was 
next  undertaken  in  this  oldest  stage.  Beginning  on  the  posterior  surface 
and  on  the  papillse,  we  find  the  layer  thickest  here  and  composed  of  at  least 
three  distinguishable  materials.  One  was  a  dense  material  which  was 
reticular  in  structure  and  stained  with  chromatic  stains  deeper  than  almost 
any  other  pure  cytoplasm  that  I  know  of.  This  material  was  one  con- 
stituent, while  the  other  substance  composing  the  general  field  at  this  point 
was  a  far  lighter  staining  material  which  was  homogeneous  and  clear.  This 
latter  seemed  to  bear  the  same  relation  to  the  denser  material  that  the 
"nuclear  sap"  does  to  a  linin  alveolum  or  reticulum  in  the  nucleus.     Like 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus.  179 

the  linin  reticulum,  this  denser  material  was  more  refractive  and  took  more 
of  the  chromatic  as  well  as  more  of  the  acid  counterstains  than  the  homo- 
geneous material  did.  I  shall  adopt  Schneider's  (31)  name  of  "Linom" 
for  the  denser  substance  and  his  name  of  "  Hyalom  "  for  the  clearer  and  more 
homogeneous  material.  It  seems  probable  from  the  works  of  Biitscheli  (9), 
Rhumbler  (27),  Hardy  (35),  Wilson  (34),  Andrews  (i),  Strasburger  (32),  and 
others  that  these  structures  of  cytoplasm,  as  seen  under  the  microscope  in 
fixed  material,  do  not  represent  the  exact  condition  as  it  exists  in  life. 
After  reading  the  pages  of  my  recent  work  on  the  electric-motor  cells  of 
Torpedo,  one  can  more  easily  see  that  the  cytoplasmic  linom  is  a  structure 
which  depends  upon  the  fixative  used  as  one  factor  and  upon  the  chemical 
and  physical  peculiarities  and  the  contents  of  this  plasma  at  the  time  that 
the  fixative  is  applied,  as  another  set  of  factors.  Its  reticular  or  alveolar 
arrangement  can  most  certainly  be  immensely  varied,  and  all  these  artificial 
conditions  must  be  very  much  different  from  that  which  obtains  in  life. 
Since  I  have  only  a  few  fixed  specimens  to  discuss,  over  whose  earlier  prepa- 
ration I  had  no  control,  I  shall  not  try  to  solve  the  question  of  what  the 
structure  was  during  life,  but  will  merely  describe  the  present  specimen  as 
it  appears. 

The  linom  of  the  cytoplasmic  layer  on  the  posterior  end  is  very  fine  and 
can  only  be  seen  with  the  best  lenses  and  under  the  best  conditions.  This 
holds  particularly  for  the  outer  portion  of  the  layer,  for  as  we  examine 
the  inner  portion  the  reticulum  grows  larger  meshed  until,  at  its  point  of 
contact  with  the  fibrillar  core,  the  meshes  are  quite  easily  seen. 

The  same  is  true  as  we  examine  this  layer  in  a  more  anterior  position. 
Here  all  the  meshes  are  proportionately  larger,  until  in  the  layer,  as  found 
covering  the  extreme  anterior  surface,  the  meshes  of  the  linom  are  visible 
with  ordinary  high  powers.  I  do  not  refer  to  the  vacuoles  which  are  found 
at  various  points,  for  there  is  a  great  difference  between  the  largest  meshes 
and  the  vacuoles,  although  the  vacuole  may  be  derived  from,  or  originate 
in,  an  overgrown  mesh.  The  meshes  always  contain  the  hyalom,  while 
the  vacuoles  do  not.  My  definition  of  a  vacuole  in  this  case  will  be  a  space 
in  the  linom  into  which  the  h^'alom  does  not  extend.  Also,  the  rounded 
outline  of  a  vacuole  shows  a  surface  tension  between  its  content  and  the 
cytoplasm,  which  does  not  seem  to  exist  between  a  mesh  of  the  linom  and 
its  contained  hyalom.  These  vacuoles  appear  to  have  contained  a  soluble 
substance  during  life,  and  the  hyalom  is  not  soluble  in  fluids  used  to  prepare 
the  specimen.  Such  vacuoles  are  found  at  various  points  in  the  cytoplasmic 
layer,  most  often  at  its  outer  edge,  while  the  large  meshes  appear  at  the 
inner  edge  and  around  the  nuclei.  The  vacuoles  become  so  large  and 
numerous  at  the  anterior  end  of  a  stage  such  as  figure  18,  plate  6,  that  the 
cytoplasm  looks  like  foam  containing  the  nuclei  and  surrounding  the 
unchanged  fibrillar  core. 

In  addition  to  this  structural  basis  of  linom  and  hyalom,  another  very 
prominent  content  of  the  cytoplasm  is  a  series  of  granules  of  some  material 


I  So  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

that  is  denser  and  somewhat  more  stainable  than  either  of  the  others. 
These  granules  are  very  numerous  and  fine  at  the  periphery,  where  they 
cause  the  outer  part  of  the  layer  to  stain  deepest  with  eosin,  and  they  lie 
in  the  finer-meshed  linom,  and  grow  larger  and  fewer  as  one  examines  the 
inner  parts  of  the  layer.  They  are  of  a  slightly  more  refractive  quality 
than  the  linom  and  stain  deeper  with  chromatic  dyes  the  larger  they  get. 
The  largest  also  have  some  color  of  their  own,  a  slight  golden-brownish  color, 
somewhat  like  that  of  pigment  granules.  While  the  smallest  granules  seem 
to  lie  in  or  attached  to  the  strands  of  linom,  the  larger  seem  to  lie  in  its 
meshes  in  the  hyalom.  They  remind  the  observer  of  the  granules  found  in 
certain  other  cells,  particularly  in  the  electric-motor  cells  of  Torpedo,  as 
well  as  in  other  nerve-cells.  In  figure  i6,  plate  5,  these  finest  granules  are 
seen  in  the  outer  part  of  the  layer  stained  with  eosin,  while  the  inner  part 
does  not  show  the  larger  ones  that  lie  there. 

The  larger  granules  are  prone  to  gather  about  the  nuclei  and  about  the 
inner  part  of  the  electric  layer,  as  seen  in  figure  22,  plate  8,  where  they  took 
the  iron  haematoxylin  stain  well.  The  larger,  chromatic-staining  granules 
are  also  found,  in  groups  or  singly,  in  many  parts  of  the  fibrillar  core. 
While  the  larger  ones  found  in  the  core,  around  the  nuclei,  and  at  the  inner 
edge  of  the  cytoplasmic  layer  seem  to  stand  in  sharp  contrast  to  the  more 
numerous  and  finer  ones  found  in  the  outer  part  of  the  cytoplasm,  one  can 
trace  steps  between  the  two  kinds. 

The  writer  believes  these  granules  to  be  the  secreted  or  prepared  nitro- 
genous material  used  by  katabolic  processes  to  produce  electricity,  either 
directly  or  indirectly.  They  must  be,  physiologically,  the  same  granules 
described  by  Ballowitz  (5)  in  the  cytoplasmic  layer  of  the  electroplax  of 
Raja,  and  by  Schlichter  (30)  in  the  cytoplasmic  regions  on  both  surfaces  of 
the  electroplax  of  Mormyrus.  In  this  latter  they  were  very  large  and  not 
easily  demonstrated.  The  writer  has  seen  some  indication  of  them  in 
Astroscopus  (12),  but  they  would  seem  to  be  noticeably  absent  in  other 
forms,  as  Torpedo  (although  they  have  been  figured  here  by  Fritsch  (20)), 
and  in  Gymnotus  (3)  and  Malopteriiriis  (6) ,  where  such  granules  as  Ballowitz 
has  described  or  figured  would  seem  to  be  inadequate  in  size  and  number 
for  so  heavy  a  duty.  It  will  be  noted  in  the  above  list  that  the  weak- 
electric  fish  seem  to  have  these  cytoplasmic  granules  best  developed,  while 
the  strong-electric  fish  show  least  of  them.  It  is  possible  that  these  latter 
possess  them,  or  their  equivalent,  in  a  more  fluid  and  less  visible  form. 

The  cytoplasmic  layer  is  marked  off  from  the  fibrillar  core  by  a  very 
definite  and  sharp  line.  It  can  not  be  said  that  a  definite  membrane 
exists  here,  although  one  may  exist.  The  boundary  between  the  outer 
layer  and  the  core  is  not  an  even  or  a  straight  one,  but  shows  a  wandering 
course,  especially  at  the  two  ends.  At  some  points  strands  of  the  linom 
seem  to  branch  into  the  core  (fig.  16,  plate  5).  At  the  anterior  end  in 
particular  it  shows  many  extensive  and  complicated  invaginations  of  its 
granule-containing  substance  into  the  fibrillar  core.     Certain  small  portions 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus.  i8i 

of  it  also  have  been  permanently  left  in  the  main  body  of  the  core,  usually 
near  its  anterior  end.  These  inclusions  (fig.  20,  plate  7)  may  or  may  not 
include  the  nuclei.     They  always  contain  some  of  the  largest  of  the  granules. 

The  nuclei  do  not  show  any  internal  peculiarities  which  would  dis- 
tinguish them  as  electric  nuclei  from  some  of  the  other  tissue  nuclei,  par- 
ticularly from  muscle  nuclei,  which  they  much  resemble.  They  have  a 
large  chromatic  content  and  a  particularly  large  plasmosome  which  stains 
deeply.  They  can  be  sharply  distinguished  from  some  other  nuclei,  as 
connective-tissue  nuclei  for  instance,  where  the  more  delicate  outline  and 
diiTerent  chromatic  pattern  is  discernible  at  a  glance. 

Each  nucleus  shows  some  sign  of  a  surrounding  differentiated  layer  of 
cytoplasm.  This  consists  of  larger  granules  and  a  zone  in  which  the 
hyloplasm  seems  to  be  in  greater  proportion  than  elsewhere.  At  different 
places  in  a  preparation  one  may  see  more  or  less  of  a  contraction  zone 
around  the  nuclei.  While  this  may  be  a  physiological  condition,  it  is  more 
probably  an  artifact  due  to  the  fixing  or  hardening. 

One  interesting  condition  is  to  be  seen  in  most  of  the  few  nuclei  which 
become  detached  from  the  outer  layer  and  included  with  some  small  portion 
of  the  outer  cytoplasm  in  the  fibrillar  core.  These  nuclei  probably  become 
so  placed  during  a  very  early  stage,  and  the  further  they  are  separated  from 
the  layer  to  which  they  rightly  belong,  the  larger  they  grow  and  the  more 
diffuse  their  chromatin  becomes.  The  plasmosome  diminishes  in  size  and 
the  whole  structure  looks  more  like  a  connective-tissue  nucleus,  except 
that  it  is  very  much  larger.  I  have  seen  this  same  condition  in  the  electro- 
plax  of  Raja. 

It  was  not  possible  to  find  a  real  electrolemma  or  cell-membrane  covering 

this  electroplax.     A  connective-tissue  covering,  more  or  less  closely  applied 

to  the  surface,  was  always  present,  but  the  fact  that  this  covering  possessed 

its  own  nuclei  seems  proof  that  it  was  a  real  connective-tissue  covering  and 

not  any  product  of  the  activity  of  the  electroplax  tissue.     At  such  points, 

as  this  connective  tissue  did  not  actually  touch  the  electroplax,  a  careful 

examination  was  made  to  see  if  some  actual  cell-membrane  did  exist. 

Beyond  the  fact  that  the  outer  edge  of  the  electric  layer  was  sharply  defined 

and  that  its  surface  was  rounded  and  even  as  if  some  membrane  was  present, 

no  real  membrane  could  be  demonstrated,  either  by  its  refractive  properties 

or  by  its  color. 

INNERVATION. 

A  general  survey  of  the  innervation  is  desirable,  as  too  little  exact  topo- 
graphical work  has  been  done  on  those  fishes  in  which  the  electric-motor 
centers  are  thinly  distributed  in  character  over  large  areas  of  the  cord,  as, 
for  instance,  in  Gymnotus,  Raja,  and  the  mormyrids.  Regions  D  and  E 
were  selected  in  the  42-day-old  embryo  as  the  most  favorable  parts  to  study. 
The  work  was  not  as  exact  as  it  could  be  if  the  investigator  had  had  plenty 
of  live  material,  especially  adult  material,  on  which  to  use  some  of  the  well- 
known  neurological  methods.     But  even  in  this  embryo,  which  was  well 


Papers  from  the  Marine  Biological  Laboratory  at  Toriugas. 


fixed  in  sublimate-acetic,  much  could  be  accurately  made  out,  and  it  is 
hoped,  too,  that  the  description  will  prove  suggestive. 

The  motor  cells  were  first  looked  for  in  the  spinal  cord,  especially  that 
region  which  was  posterior  and  in  the  neighborhood  of  the  electric  organs. 
Assuming  for  the  present,  as  a  law,  that  electric-motor  cells  are  larger  than 
muscle-motor  cells  which  innervate  an  equal  weight  of  muscle,  it  was  very 
easy  to  find  groups  of  large,  heavy  nerve-cells  scattered  through  the  sub- 
stance of  the  spinal  cord  from  near  the  anterior  beginning  of  the  spindles 
to  the  very  last  point  in  the  tail  to  which  they  extend. 


"iG.  8. — Side  view  of  reconstruction  (semi- 
diagrammatic)  of  spinal  cord  and  motor 
electric  nerves  of  a  larval  Gymnarchns 
42  days  old.  Arrow  indicates  anterior 
and  posterior  directions.  Ni,  N2,  Nz, 
and  Ni  are  the  four  electric  nerves  formed 
by  branches  from  motor  roots  of  spinal 
nerves.  E  indicates  small  branches  from 
electric  nerves  that  innervate  posterior 
surfaces  of  electroplaxes.  g  marks  spinal 
ganglion,  whose  afferent  nerves  have 
been  cut  off. 


E  jr~  r 


^^ir~      rsc 


These  cells  were  situated  just  above  the  central  canal  and  from  i  to  4, 
or  even  5,  could  be  seen  in  every  transverse  section  in  most  of  this  length 
(fig.  14,  plate  i[,  A,  B,  and  C).  They  did  not  appear  to  be  divided  into  two 
symmetrical  groups,  but  rather  to  lie  in  one  median  group.  On  the  one 
hand,  their  dorsal  position  might  appear  to  be  evidence  that  they  were  not 
motor-cells  or  that  they  were  not  to  be  considered  as  modified  muscle-motor 
cells.  On  the  other  hand,  the  presence  of  real  ventral  muscle-motor  cells 
(fig.  14,  plate  4,  D  and  E)  in  the  whole  length  of  the  cord,  the  correspondence 
of  the  cells  under  discussion  with  the  position  of  the  electric  spindles,  and 
the  fact  that  Fritsch  describes  similarly  placed  cells  in  the  same  position 
in  mormyrids  as  the  motor-nerve  cells  of  the  electric  organ,  all  seemed  to 
constitute  very  strong  indirect  evidence  that  these  were  the  nerve  cells 
which  innervated  the  posterior  ends  of  the  electroplaxes. 

In  addition,  the  writer  was  able  to  trace,  in  a  number  of  cases,  the  course 
of  the  neuraxons  through  the  cord,  out  into  the  ventral  roots  of  the  nerves, 
and  finally  into  special  bundles  of  nerve  fibers  that  undoubtedly  innervated 
the  electric  tissue.  No  one  nerve  process  was  actually  traced  for  the  v/hole 
distance  unbroken,  but  the  various  parts  and  regions  were  so  pieced  together 
that  the  course  was  well  established  and  corresponds  in  many  ways  with 
Fritsch 's  observations  on  Mormyrus.  Text-figure  9  shows  a  semidiagram- 
matic  cross-section  of  these  courses  as  seen  from  in  front,  while  text-figure  8 
shows  the  same  thing  sketched  in  relief  from  the  side.  This  topography 
will  presently  be  described. 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus. 


The  electric-motor  cells  at  this  age  must  of  course  be  considered  as  still 
very  young,  and  descriptions  from  the  adult  are  desirable.  We  will  begin 
by  examining  the  9-day-old  embryo  once  more  to  see  if  they  have  started. 
In  this  cord  (fig.  8,  plate  2)  there  are  but  very  faint  traces  of  any  nerve- 


FiG.  9. — Diagram  to  show  position  and  relations  of 
nerve  elements  to  electric  spindles.  S.C.,  spinal 
cord,  showing  central  canal  and  four  motor  elec- 
tric nerve-cells.  Processes  from  these  cells  pass 
out  through  ventral  roots,  and  distribution  of  these 
roots  to  electric  spindles  and  muscle-masses  is  in- 
dicated. M ,  some  of  the  muscle-masses.  D,  U.M., 
■  L.M.t  and  V  show  electric  spindles  on  one  side. 
Sp.G.,  spinal  ganglion;  L.L.N,,  lateral  line  nerve. 


cell  development,  and  some  neuroblast  mitosis  is  still  going  on.  In  the 
position  to  be  later  occupied  by  electric  cells  a  little  enlargement  of  nucleus 
and  cell-body  is  visible.  One  cell  here  is  of  great  interest,  and  that  is  one 
of  the  now  well-known  "  Hinterzellen "  or  giant  cells,  first  described  by 
J.  Beard  (37),  Rohon  (29),  Studnitzka  (33),  and  others,  in  the  embryos  of 
Salmo  and  Raja,  and  later  by  Fritsch  (21),  as  a  different  sort  of  cell,  in  the 
adult  LopJiius.  Still  later,  such  cells  were  described  by  the  writer  (36)  in  the 
embryos  and  adults  of  various  Pleuronectidae  and  in  Pterophryne,  where  he 
showed  it  to  be  the  same  cell  in  both  embryo  and  adult,  and  described  the 
relations  of  anterior  and  posterior  branches  of  the  neuraxon.  From  its 
size,  shape,  and  position  in  the  present  specimen,  it  seems  that  this  dorsal 
cell  might  be,  in  some  way,  related  to  the  electric-motor  cells,  but  that 
question  is  easily  settled  when  one  examines  the  12-day  embryo  and  finds 
that  all  of  the  dorsal  cells  have  effectually  disappeared  before  the  electric- 
motor  cells  begin  to  differentiate.  Besides,  the  well-known  fiber  courses  of 
the  dorsal  cells  as  worked  out  by  Fritsch  in  LopJiius,  the  writer  in  Ptero- 
phryne, Harrison  in  Salmo,  and  Johnson  in  Catostomus  are  sufficient  proof 
that  the  two  have  nothing  in  common. 


1 84  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

The  42-day  larva  shows  the  best  development  of  these  electric-motor- 
nerve  cells  until  an  older  fish  can  be  described.  Here  we  find  a  heavy, 
rounded,  cytoplasmic  body  of  about  40  microns  in  diameter  (fig.  14,  plate  4, 
A,  B,  and  C).  Its  outline  is  usually  pear-shaped,  with  the  axis-cylinder 
process  given  off  at  the  pointed  end.  Dendrites  are  not  visible  in  the 
specimen  at  hand.  While  most  of  these  cells  are  of  the  same  size,  a  few 
are  noticeably  smaller. 

The  nucleus  is  large,  round,  and  placed  nearly  always  in  a  very  eccentric 
position.  This  position  is  most  frequently  on  the  side  away  from  the 
neurite,  but  sometimes  it  may  be  very  close  to  the  neurite.  Its  diameter 
is  about  16  to  19  microns  in  the  largest  cells  of  this  age,  and  its  outline  is 
hard  and  round.  The  nucleolus  consists  of  a  single,  or  rarely  double,  plas- 
mosome,  which  is  a  very  little  less  than  5  microns  in  diameter,  but  which, 
when  double  or  multiple,  is  of  proportionally  smaller  size.  As  I  have  shown 
in  the  nucleus  of  the  electric  cells  of  several  torpedoes,  when  a  plasmosome 
is  multiple  its  several  parts,  collectively,  are  larger  in  mass  than  is  a  single 
normal  or  usual  plasmosome. 

Of  course,  the  nucleus  was  carefully  examined  as  to  any  possible  ori- 
entation of  its  nuclei,  particularly  the  plasmosome,  with  reference  to  gravity 
or  to  the  electric  current  or  to  the  axis  of  the  cell.  Nothing  of  this  sort 
could  be  found,  although  this  does  not  preclude  such  a  condition  in  the 
grown  fish.  It  is  known  that  in  Torpedo,  where  such  an  orientation  does 
exist,  this  same  orientation  is  not  found  in  the  embryonic  or  larval  stages. 

From  these  cells  thin,  delicate  axis-cylinder  processes  were  traced  down, 
as  has  been  described,  and  into  the  ventral  roots  of  the  spinal  nerves  in  a 
sufficient  number  of  cases  to  assure  the  observer  that  they  all  ran  in  this 
direction.  The  process  was  thinnest  shortly  after  leaving  the  cell,  and 
became  thicker  as  it  approached  the  nerve-root.  When  it  once  entered  the 
root  it  again  became  very  thin,  although  now  invested  with  a  connective 
tissue  and  a  medullary  sheath.  A  large  number  of  connective- tissue  nuclei, 
nerve-sheath  nuclei,  and  some  unknown  elements  cause  the  motor-root  of 
the  nerve  to  swell  to  some  size  just  after  leaving  the  cord.  It  decreases 
again  in  size  before  it  enters  the  foramen  and  leaves  the  vertebral  canal  in 
company  with  the  much  smaller  dorsal  root. 

Just  outside  the  canal  the  dorsal  root  ends  in  the  spinal  ganglion,  and 
the  motor-root  traverses  the  inner  side  of  the  ganglion,  from  which  it  emerges 
on  the  lower  edge,  and  at  once  divides  into  a  dorsal  and  a  ventral  branch. 

The  ventral  branch  passes  backward  and  downward  (see  text-figs.  8 
and  9)  to  a  point  at  a  level  with  the  lower  middle  spindle,  and  here  it  gives 
off  a  considerable  group  of  fibers  which  pass  caudad  just  inside  of  the 
spindle  capsule.  These  fibers  are  joined  by  similar  groups  from  others  of 
the  spinal  nerves  and  the  whole  mass  forms  the  lower  middle  electric  nerve 
(text-fig.  8,  TVs).  At  the  posterior  level  of  each  electroplax  this  nerve  gives 
off  a  few  fibers  (text-fig.  8,  E)  which  branch  out  and  innervate  the  posterior 
surface  of  this  electroplax. 


Origin  of  Electric  Tissues  of  Gynmarchus  Niloticus.  185 

The  remainder  of  the  ventral  branch  passes  farther  down  and  again  gives 
off  a  branch  which  goes  to  form  part  of  the  ventral  electric  nerve  (text-fig. 
8,  N^.  This  latter  sends  off  little  branches  to  furnish  the  posterior  surfaces 
of  the  ventral  electroplaxes  with  motor-fibers  (text-fig.  8,  £). 

Going  back  to  the  anterior  root,  we  find  that  its  dorsal  branch  leads 
directly  upward  and,  passing  between  the  spinal  ganglion  and  neural 
arch,  slopes  gently  backward  to  give  off  a  large  branch  which  becomes  a 
part  of  the  upper  median  electric  nerve  (text-fig.  8,  iVj).  Its  remaining 
fibers  reach  upward  and  furnish  the  dorsal  spindle  with  a  part  of  the  fibers 
that  form  its  dorsal  electric  nerve  (text-fig.  8,  iVi). 

Of  course,  there  are  muscle-motor  elements  in  both  these  branches  of 
the  anterior  nerve-root,  the  number  depending  upon  the  position,  back- 
ward or  forward,  at  which  we  examine  the  arrangement.  Two  examples  of 
the  motor-nerve  cells  of  the  muscle-tissue  from  the  cord  in  region  D  are 
shown  in  figure  14,  plate  4,  D  and  E.  At  the  level  of  the  anterior  parts  of 
the  electric  organ,  when  there  are  large  quantities  of  muscle  and  the  electro- 
plaxes are  very  small,  the  muscle  branches  are  large,  particularly  the  dorsal 
branches,  which  have  to  supply  the  muscle-bundles  for  the  large  dorsal  fin. 
In  the  posterior  region  of  the  tail,  on  the  other  hand,  the  muscle  is  almost 
entirely  absent  and  the  muscle-branches  of  the  anterior  roots  are  not  even 
easily  seen. 

The  eight  (four  on  each  side)  longitudinal  electric  nerves  are  interest- 
ing in  that  they  form  a  morphological  buffer  between  the  conflicting 
segmentation  of  the  nervous,  skeletal,  and  muscular  systems,  and  the  inde- 
pendent segmentation  of  the  electric  system.  Were  the  electric  segmenta- 
tion to  correspond  with  that  of  the  others,  we  should  not  find  these  nerves 
in  this  recognizable  form.  In  fact,  in  the  anterior  part  of  the  electric  organ 
we  do  not  find  them  as  continuous  nerves,  in  many  places,  for  more  than 
two  or  three  neuromeres  at  a  time.  And  even  posteriorly  where  they  do 
form  continuous  nerves,  the  fibers  that  enter  them  from  any  given  spinal 
nerve  do  not  pass  very  far  back  in  them  before  leaving  to  innervate  one  of 
the  electroplaxes.  Each  nerve  cell  probably  lies  but  a  very  short  distance 
in  front  of  the  electroplax  which  it  supplies.  This  was  decided  upon  by 
plotting  the  relative  positions  of  all  motor-electric  cells  and  all  electroplaxes 
in  the  larger  part  of  the  organ. 

While  doing  this  it  was  also  determined  how  many  nerve-cells  sent 
their  nerve-processes  to  each  electroplax.  Thus,  in  region  E  of  the  42- 
day-old  larva  there  were  easily  counted  414  of  the  electric-motor  cells,  while 
in  the  same  part  there  were  81  electroplaxes.  This  makes  it  quite  sure 
that,  on  an  average,  about  5  of  the  nerve-cells  were  used  for  each  electro- 
plax. It  was  attempted  to  count  the  nerve-fibers  as  they  left  the  electric 
nerve  to  branch  out  over  the  electroplax,  but  the  elements  were  too  small 
and  the  fixation  not  just  what  was  needed  to  do  this.  It  could  be  easily 
done  in  a  grown  specimen. 

I  shall  now  take  up  the  structure  of  the  nerve-fibers  as  they  leave  the 


i86  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

electric  nerves  to  pass  to  the  electroplaxes,  assuming  that  they  have  no 
peculiarities  of  interest  during  their  course  through  the  nerve-tracts  between 
the  cord  and  the  electric  chambers.  (The  electric  chambers  are  not  divided 
as  in  Mormynis  by  a  transverse  septum  of  heavy,  opaque,  white  connective 
tissue;  this  can  be  explained  by  the  fact  of  their  secondary  segmentation.) 

As  the  few  fibers  destined  for  any  particular  electroplax  first  leave  the 
electric  nerve  they  are  directed  caudad  and  are  very  small  in  diameter  and 
invested  with  a  fine  connective-tissue  sheath.  This  covering  is  probably 
medullated  in  the  grown  fish.  They  quickly  turn  in  a  gentle  curve  whose 
diameter  is  about  half  that  of  the  spindle  and  pass  forward  through  the 
electric  connective  tissue  to  the  posterior  surface  of  the  electroplax.  At  the 
beginning  of  this  curve,  or  just  after  leaving  the  electric  nerve,  the  axis 
cylinder  or  neuraxon  enlarges  to  a  considerable  diameter  and  becomes  very 
wavy  and  irregular  in  diameter.  Its  course  is  no  longer  straight,  and  it  is 
not  possible  to  find  a  section  of  any  considerable  part  of  its  length,  except 
in  some  very  few  cases.  Its  sheath  of  connective  tissue  is  loose  fitting  and 
the  inner  side  shows  a  loose  reticulum  of  fine  fibrils  and  plates  that  form  a 
weak  connection  between  the  axon  and  sheath. 

It  is  at  once  apparent  in  the  maze  of  fibers  which  approach  the  electro- 
plax that  the  few  neuraxones  which  first  entered  the  compartment  are  now 
many  in  number.  Still,  it  is  difficult  to  see  where  they  branch  in  the  thin 
sections,  owing  to  their  sinuous  and  irregular  courses  and  to  the  large 
numbers  of  transverse  and  oblique  sections  present.  In  several  places  this 
branching  was  seen,  however,  and  recognized  as  the  same  multiple  branching 
which  has  often  been  described  in  the  terminal  part  of  nerve-paths  and,  in 
particular,  in  the  same  comparative  region  of  the  electric  tissue  of  Raja  by 
Ballowitz  (5)  and  Retzius  (26) ,  in  Torpedo  by  several  authors,  and  especially 
in  Malopterurus  by  Ballowitz  (4),  who  refers  in  this  article  to  many  other 
cases.  The  writer  has  also  seen  and  figured  it  in  the  electric  tissue  of 
Asiroscopus  in  a  paper  soon  to  appear.  In  the  present  case,  one  section,  as 
can  be  seen  in  figure  16,  plate  5,  exhibits  two  cases  of  this  branching,  one  of 
them  showing  a  single  fiber  dividing  into  at  least  three  or  four  branches; 
also,  figure  25,  plate  9,  in  which  there  is  to  be  seen  a  well-defined  single  fiber 
dividing  into  two  branches  just  before  they  end  in  two  club-shaped  nerve- 
endings  in  the  electric  layer  of  the  posterior  surface  of  an  electroplax. 
The  abundant  nodes  of  Ranvier,  described  by  Schlichter  (30)  in  the  case 
of  Mormyrus,  were  not  observable  here,  probably  on  account  of  the  lack  of 
osmic-acid  fixation.  I  have  no  doubt  that  they  are  present  and  they  must 
be  present  at  the  points  at  which  the  fibers  divide. 

As  has  already  been  stated,  the  fibers  now  approach  and  end  on  the 
posterior  surface  of  the  electroplax,  as  well  as  on  the  lower  sides  of  some  of 
the  papillae  that  arise  from  it.  The  mode  of  ending  is  not  difficult  to  see, 
apparently,  although  this  much  studied  and  controverted  question  should 
not  be  approached  lightly,  especially  where  the  material  is  merely  a  subli- 
mate-acetic fixation  stained  with  iron-heematoxylin  and  eosin. 


Origin  of  Electric  Tissues  of  Gymnarchtis  Niloticus.  187 

The  nerve-process,  carrying  its  connective- tissue  sheath  until  it  actually 
reaches  the  surface  of  the  electroplax,  ends  in  a  blunt  and  somewhat  thick- 
ened knob  which  is  embedded  in  a  hollow  or  invagination  in  the  substance 
of  the  posterior  cytoplasmic  or  electric  layer  of  that  structure  (see  figs.  18 
and  19,  plate  6;  also  fig.  16,  plate  5;  also  figs.  24  and  25,  plate  9).  This  knob 
may  be  quite  elongate  in  form  and  in  some  cases  appears  to  be  branching. 
The  nerve  fiber  becomes  very  narrow  and  apparently  dense  just  before 
entering  the  cavity,  but  it  quickly  broadens  out,  to  as  much  as  or  more  than 
its  previous  width,  to  fill  the  cytoplasmic  cavity  of  the  electroplax.  Its 
substance  becomes  very  light-staining,  more  so  than  any  other  part  of  its 
length  is,  and  this  light-staining  quality  is  most  apparent  at  its  extreme 
distal  end  in  the  cavity.  The  nerve-fibrils,  faintly  visible  in  the  outer 
courses  of  the  axon  and  rather  more  so  in  the  denser  neck  just  before  entering 
the  cavity,  can  be  seen  in  the  light-colored  club-shaped  ending  to  be  running 
in  an  irregular  reticulum  instead  of  in  their  previously  almost  straight  and 
parallel  manner.  They  could  not  be  traced  into  the  protoplasmic  bridges 
between  the  club-shaped  nerve-ending  and  the  surface  of  the  protoplasmic 
cavity  in  which  it  lies.  In  very  few  instances  did  the  nerve-substance  fill 
the  recess  in  the  electric  layer  tightly,  probably  owing  to  some  shrinkage  in 
both  of  the  tissues.  The  small  space  between  the  two  surfaces  is  crossed 
by  the  numerous  fine  processes  or  strands,  mentioned  above,  of  some  of  the 
nerve-tissue  remnants,  probably  of  an  original  closer  contact. 

The  question  now  arises  as  to  whether  the  cavity  or  depression  which 
the  nerve-ending  occupies  is  to  be  considered  as  an  invagination  of  the 
surface  of  the  electric  layer  or  as  a  real  penetration  of  the  electroplax  by 
the  nerve.  I  shall  consider  it  as  the  former,  because  the  surface  of  the 
electroplax  appears  not  to  be  interrupted  by  the  opening  but  to  continuously 
follow  the  inner  edge  of  the  cavity  all  the  way  around. 

The  presence  of  a  perceptible  cell-membrane  or  electrolemma  would 
assist  in  the  solution  of  this  question,  but  such  an  organ  could  not  be 
demonstrated.  The  edge  of  the  cytoplasm  was  sharp  and  definite,  but  no 
membrane  was  visible,  either  by  its  staining  properties  or  by  refrangibility. 
It  possibly  will  be  found  in  the  adult  organ.  Nor  was  it  possible  to  see  the 
"Stabchen"  or  rodlets  which  have  been  described  in  other  electric  organs 
by  Ballowitz  (3,  5)  and  his  pupils  (30).  Some  striated  arrangement  of 
the  granular  electrochondria  or  granules  described  previously  was  observ- 
able, but  this  constituted  a  fibrillar  secretory  striation,  such  as  is  seen  in 
the  surfaces  of  most  cells  that  are  undergoing  exchanges  of  any  kind.  Here, 
again,  we  must  await  examinations  of  the  adult  electroplax  before  we  can 
say  if  such  "Stabchen"  are  present;  also  if  they  are  homologous  with  the 
well-defined  rodlets  found  by  Ballowitz  in  Torpedo  and  in  Raja,  and  whether 
they  are  specific  structures  of  any  importance  in  the  production  of  electricity. 

Returning  to  the  relation  of  nerve-ending  to  electroplax,  we  have 
practically  decided  that  the  nerve-club  is  embedded  in  an  invagination  of 
the  surface  rather  than  in  a  cavity  in  the  substance  of  the  electric  layer. 


l88  Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

In  doing  this  it  has  formed  a  much  more  intimate  connection  than  may 
be  seen  in  other  places  where  the  nerve-fibers  touch  the  electroplax  very 
closely,  even  being  partly  embedded  in  it,  or  running  through  the  fundus 
that  lies  between  two  papillse,  where  the  fiber  is  closely  pressed  on  three 
sides  by  the  surface  of  the  electroplax.  In  these  fiber  contacts  the  con- 
nective-tissue sheath  persists,  while  in  the  club-shaped  or  heavily  rounded 
endings  the  connective-tissue  sheath  is  lost,  lefc  at  the  surface,  and  the 
little  protoplasmic  bridges,  shown  when  slight  shrinkage  has  taken  place, 
testify  to  the  intimacy  of  the  contact.  In  addition  a  slight  amount  of  fine, 
golden-colored  granules  surrounds  the  nerve-ending,  Ij'ing  on  its  surface, 
between  it  and  the  substance  of  the  electroplax  (figs.  24  and  25,  plate  9). 
These  granules  are  not  found  on  any  other  part  of  the  nerve  surface. 

Naturally  the  paper  of  Schlichter  (30)  was  carefully  examined  to  see 
what  he  had  found  as  to  the  ending  of  the  electric  nerves  on  the  related  form, 
Mormyrus  oxyrhynchus.  He  had  adult  material,  but  otherwise  was  no 
better  off  than  the  writer  in  the  possession  of  material  which  had  been 
treated  especially  for  neurological  study.  He  describes  the  nerve-fibers  with 
their  medullary  sheaths  as  coming  in  contact  with  the  large  process  of  the 
electroplax  and  then  suddenly  ending  just  as  they  reach  certain  large  inden- 
tations of  the  surface  of  this  electroplax.  He  found  in  these  indentations 
only  a  little  coagulated  material  and  some  nuclei. 

The  writer  has  no  doubt  that  the  slight  coagulum  represents  what 
remains  of  a  club-shaped  nerve-ending  similar  to  that  which  he  finds  in 
Gymnarchus.  The  nuclei,  from  their  position  in  Schlichter's  picture,  are 
evidently  the  nuclei  of  terminal  connective- tissue  coverings.  If  this  idea 
be  correct  we  will  have  a  very  simple  but  interesting  form  of  nerve-ending, 
much  larger  in  size  than  that  found  on  any  muscle  or  any  other  electric 
organ  and  one  in  which  it  will  be,  apparently,  easier  to  study  the  intimate 
contact  of  nerve-substance  with  motor-substance  from  a  cytological  point 
of  view  than  in  any  other  form.  In  particular,  we  should  try  to  stain  these 
endings  with  the  nitrate  of  silver  and  methylene  blue  methods  devised  for 
neuro-cytological  studies.  This  work,  however,  can  be  undertaken  only 
on  the  ground,  with  good  laboratory  facilities  and  with  an  abundance  of 
fresh  material. 

The  writer  has  published  observations  on  some  peculiar  horizontal, 
pointed  rods,  or  pointed  threads,  found  imbedded  in  the  electric  layer  of 
the  electroplax  of  Aslroscopus.  At  that  time  he  suggested  that  they  might 
be  in  some  way  homologous  with  or  related  to  the  "Stabchen"  because  of 
the  absence  of  any  other  well-defined  "Stabchen"  in  this  fish.  Such 
structures  are  not  found  in  the  present  Gymnarchus  larva,  but  they  have 
been  seen  and  described  in  Raja,  in  a  paper  soon  to  be  published.  Their 
presence  in  Raja,  in  addition  to  the  "Stabchen,"  proves  them  to  be  entirely 
different  cell  organs. 

One  word  in  regard  to  certain  posbibilities  for  the  physiological  study 
of  the  electric  organ  in  Gymnarchus.     A  recent  paper  by  Bernstein  and 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus.  189 

Tschermak  and  another  by  Bethe  go  to  show  that  the  electric  discharge  of 
Torpedo  and  other  fishes  is  produced  by  different  concentrations  of  sodium 
chloride  in  the  electroplaxes  and  in  the  intervening  electric  connective  tissue. 
Since  there  is  a  long  series,  theoretically,  of  these  alternate  segments  of 
higher  and  lower  degrees  of  concentration  of  the  electrolyte,  and  since  all  the 
higher  concentrations  are  presumably  equal,  and  all  the  lower  concentra- 
tions are  equal,  we  would  have  a  series  of  equal  potentials  alternately 
opposed  to  each  other,  and  the  result  would  be  zero  or  else  only  the  strength 
of  one  concentration  current,  in  case  there  was  one  more  or  one  less  of 
either  of  the  concentrations. 

To  obviate  this  difficulty  a  membrane  has  been  imagined,  on  one  surface 
of  each  electroplax,  presumably  the  electric  or  nerve-ending  surface  (pos- 
terior surface  in  this  case),  which  will  be  permeable  only  to  one  kind  of 
the  ions,  either  negative  ions  or  positive  ions,  and  by  which  the  current  is 
thus  rendered  integral  in  one  direction. 

Two  things  remain  to  be  proved  in  connection  with  the  above  theory — 
the  fact  of  different  concentrations  and  the  presence  of  such  a  membrane. 
This  can  be  done  in  any  fish  in  which  it  is  possible  to  effectively  separate 
the  segments  in  a  fresh  state,  so  as  to  submit  them  to  delicate  chemical 
tests.  In  Gymnarchus  we  have  a  fish  whose  elements  are  larger  than  those 
in  any  other  one  of  the  seven  electric  types — large  enough  to  be  cut  apart, 
I  believe,  and  analyzed  separately  in  the  chemical  laboratory;  also  large 
enough  to  submit  anterior  and  posterior  surfaces  to  physical  tests  that  may 
show  its  permeability  to  either  positive  or  negative  ions  and  its  imperme- 
ability to  the  other  kind  in  one  direction.  This  experimental  work  can 
most  certainly  be  done  if  the  proportion  between  the  bulk  of  electric  con- 
nective tissue  and  electroplax  remains  the  same  in  the  adult  as  in  the  larva 
(see  fig.  23,  plate  9).  Fritsch  shows  much  less  of  the  electric  connective- 
tissue  segments  in  his  figures  of  the  adult  organ. 

Numerous  other  anatomical  features  of  Gymnarchus  have  caused  it  to 
be  classed  with  the  other  mormyrid  fishes.  This  fact  makes  it  of  interest 
to  compare  its  electroplax  with  the  very  different  electroplax  in  these 
fishes. 

That  found  in  Mormyrus  oxyrhynchus  will  serve  as  a  type  and  its  general 
plan  has  been  well  shown  by  Ogneff  (25)  and  Schlichter  (30).  Here  it  is 
evident  that  a  number  of  consecutive  and  entire  myotomes  have  been 
converted  into  electroplaxes  and  that  the  middle  layer  of  each  electroplax 
is  composed  of  unaltered  and  clearly  striated  myofibril  bundles.  The  large 
number  of  these  fibril  bundles,  and  their  distribution,  indicate  that  the 
whole  electroplax  in  Mormyrus  is  a  syncytium  composed  of  all  or  most  of 
the  cells  which  would  otherwise  have  gone  to  make  up  the  single  myotome. 
In  this  we  find  an  agreement  with  the  electroplax  of  Gymnarchus  which  is 
also  formed  from  several  cells.  In  the  one  case,  however,  all  the  cells  in 
the  myotome  have  been  used  (Mormyrus) ;  in  the  latter  only  those  lying  in 
eight  particular  localities  (Gymnarchus).     (See  paper  by  Dahlgren  (36).) 


igo  Papers  from  iJie  Marine  Biological  Laboratory  at  Tortugas. 

Further  homology  is  seen  in  the  disposition  of  the  probably  superfluous 
myofibrils.  In  both  forms  they  are  relegated  to  a  middle  position  in  the 
electroplax,  while  the  apparently  more  important  cytoplasm  forms  layers 
on  the  anterior  and  posterior  surfaces  of  the  structure.  Also,  in  both,  the 
now  useless  myofibril  bundles  are  packed  out  of  the  way  at  right  angles  to 
the  axis  of  the  electroplax,  which  remains  the  same  as  the  former  axis  of 
the  muscle-cells  that  were  used  to  form  it. 

The  only  difierence  lies  in  the  fact  that  the  striation  of  the  fibrils  is 
retained  in  the  Mormyrus  forms,  while  it  is  lost  in  Gymnarchus,  the  dark- 
staining  anisotropic  substance  apparently  dissolving  away. 

From  what  little  can  be  predicted  concerning  the  possible  origin  of  the 
electric  tissue  in  the  other  teleost  forms,  it  is  probable  that  the  Mormyrida; 
(including  Gymnarchus)  are  the  only  fish  in  which  the  electroplax  is  formed 
as  a  syncj'tium  from  more  than  one  cell.  In  Astroscopiis,  Electrophorus 
(formerly  Gymnotus),  and  Maloptenirus  the  structures  show  every  evidence 
of  having  been  developed  from  single  myoblasts  with  the  exception  of 
Malopterurus,  v/here  it  is  a  question  as  to  whether  they  are  not  evolved  from 
gland-cells  instead. 

SUMMARY. 

In  conclusion  it  may  be  stated  that — 

(i)  The  electric  tissues  of  Gymnarchus  are  developed  by  the  differ- 
entiation of  certain  portions  of  its  normal,  striated  muscle-tissues  during 
an  embryonic  or  larval  period  extending  from  the  ninth  day  to  the  forty- 
second  day  of  embryonic  life.  The  critical  period  of  this  change  takes  place 
in  the  neighborhood  of  the  eleventh,  twelfth,  to  fifteenth  days. 

(2)  The  muscle  fibers  which  go  to  form  the  electric  tissue  give  up  their 
usual  segmentation  into  myotomes,  and  first  form  eight  long  and  con- 
tinuous spindles  which  afterward  segment,  each  into  a  lesser  number  of 
masses,  the  electroplaxes. 

(3)  The  mj'ofibrils  lose  their  transverse  striation  and  form  a  large  inner 
core  for  each  electroplax.  They  lie  in  a  wavy  mass,  mostly  at  right  angles 
to  their  former  course.  The  active  cytoplasm  forms  an  outer  layer  which  is 
denser  and  stains  deeper  on  the  posterior  than  on  the  anterior  surfaces.  It 
contains  granules. 

(4)  Each  electroplax  is  made  up  of  several  muscle  cells,  12  to  20  or  more. 
This  is  different  from  the  two  elasmobranch  fishes,  in  which  each  muscle 
cell  forms  only  one  electroplax. 

(5)  The  nerve  is  distributed  on  the  posterior  surface  and  ends  in  blunt 
knobs  that  lie  in  cavities  formed  by  the  invagination  of  the  surface  of  the 
electric  layer.  The  current  probably  runs  from  tail  toward  head,  as  in 
Mormyrus. 

(6)  The  development  of  the  tissue  gives  a  strong  clew  to  the  probable 
development  of  the  electric  tissues  in  the  other  mormyrid  fishes. 


LITERATURE. 

1.  Andrews,  Mrs.  G.  F.     The  living  substance  as  such  and  as  organism.     Journ.  of 

Morph.,  vol.  XII,  2,  Supplement. 

2.  AssHETON,  Richard.     (See  No.  24.) 

3.  B.4LL0WITZ,  E.     Zur  Anatomic  des  Zitteraales  mit  besonderer  Beriicksichtigung  seiner 

elektrischen  Organe.     Arch.  f.  Mik.  Anat.,  Bd   50,  1897. 

4.  .     Ueber  polytome  Nervenfaserteilung.     Anat.  Anz.,  Bd.  xvi,  s.  541,  1899. 

5.  .     Ueber  den  feineren  Bau  des  elektrischen  Organs  des  gewohnlichen  Rochen, 

Raja  clavata.     Anat.,  Hefte,  Merkel  und  Bonnet,  1897. 

6.  .     Das  elektrische  Organ  des  afrikanischen  Zitterwelses,  Malopterurus  electricus 

Lac.     Anatomische  Untersuchs.,  Fischer,  Jena,  1899. 

7.  BlEDERMAN,  W.     Elektrophysiologie.     G.  Fischer,  Jena,  1895. 

8.  BuDGETT,  J.  S.     (See  No.  24.) 

9.  BtJTSCHEU,  O.     Untersuchungen  fiber  mikroskopischen  Schaum  und  das  Protoplasmas. 

London,  1894. 

10.  Dahlgeen,  U.     The  giant  ganglion  cells  in  the  spinal  cord  of  the  order  Heterasomata. 

Anat.  Anz.,  Bd.  xill,  pp.  281-293. 

11.  .     A  new  type  of  electric  organ  in  an  American  teleost  fish,  Astroscopus  (Bre- 

voort).     Science,  vol.  23,  p.  469. 

12.  ,  AND  F.  W.  Silvester.     The  electric  organ  of  the  "Stargazer,"  Astroscopus 

(Brevoort).     Anat.  Anz.,  Bd.  29,  p.  387. 

13.  Englemann,  Th.  W.     Die  Blatterschicht  der  elektrischen  Organe  von  Raja  in  ihren 

genetischen   Beziehungen  zur  quergestreiften   Muskelsubstanz.     Pfiug. 
Arch.,  Bd.  57,  p.  149,  1894. 

14.  Erdl,  M..  p.     Ueber  Gymnarchus  niloliciis.     Bull.  d.  Konigl.  Bayer.  Akad.  d.  Wiss. 

(Gelehrte  Anzeiger),  No.  69.     Munchen,  Oct.,  1846. 

15.  .     Ueber  eine  neue  Form  elektrischen  Apparates  bei  (Gyinncrchiis  nilolicus). 

Idem.,  Nr.  13,  April  13,  1847. 
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179.  P-  399,  1889. 

17.  .     The  electrical  organ  of  the  skate:   Observations  on  the  structure,  relations, 

progressive  development,  and  growth  of  the  electric  organ  of  the  skate. 
Phil.  Trans.  Roy.  Soc.  of  London,  vol.  183,  pp.  398,  pi.  26-30,  1893. 

18.  Fritsch,  G.     Weiterer  Beitrage  zur  Kenntnis  der  schwach  elektrischen  Fische.     Sitz.- 

ber.  d.  Preuss.  Akad.  d.  Wiss.     Juni  bis  Dez.,  1891. 

19.  .     Zur  Organization  des  GymttarcJms  niloticus.      Sitz.  d.  konig.  preuss.  Akad.  d. 

Wiss.  zu  Berlin,  Philos.-iVlath.  Class,  von  5  Feb.,  1S85. 

20.  .     Die  elektrischen  Fische.     Leipzig,  Veit  &  Co.,  1890. 

21.  .     Ueber  einige  bemerkswerthe  Elcmente  des  Centralnervensystems  von  io^fe'Mi 

piscalorius.     Arch.  f.  Mik.  Anat.,  Bd.  27,  s.  13-31. 

22.  G.ARTEN,  S.     Die  Production  von  Elektricitat.     In  Winterstein's  Handbuch  der  ver- 

gleichenden  Physiologic.     8  bis  lote  Lieferung.     Jena,  Fischer,  1910. 

23.  IWANZOFF,  N.     Das  Schwanzorgan  von  Raja.     Bull,  de  la  Societe  imperiale  des  Nat- 

uralistes  de  Moscou,  p.  53,  1895. 

24.  Kerr,  J.  Graham.     "The  work  of  John  Samuel  Budgett."     Cambridge  Univ.  Press, 

1907.     (Articles  by  Richard  Assheton,  Budgett  and  Arthur  Shipley.) 

25.  Ogneff,  J.     Einige  Bemerkungen  fiber  den  Bau  dess  chwachen  elektrischen  Organes 

bei  den  Mormyriden.     Zeits.  f.  Wiss.  Zool.  Bd.  64,  p.  565,  1898. 

26.  Retzius,  G.     Ueber  die  Endigung  der  Nerven  in  dem  elektrischen  Organen  von  Raja 

clavata.     Biologisches  Untersuchungen,  N.  F.,  Bd.  viil,  1898. 

27.  Rhumbler,  L.     Versuch  zu  einer  mech.  Erklarung  der  indirecten    Kern    und    Zell- 

teilung.    Arch.  Entwick.  Mechanik,  Bd.  ill. 

28.  RUPPEL,  Ed.     (Reported  by  Fritsch.     See.  No.  19.) 

29.  RoHON,  V.     Zur  Histogenesis  des   Ruckenmarks  der  Forelle.     Sitz.  d.  Math.  Phys. 

Klass  ed.  Akad.  d.  Wiss.  in  Munchen,  Bd.  14,  1884. 

30.  ScHLiCHTER,  Heinrich.     Ueber  den  feineren  Bau  des  schwach  elektrischen  Organs  von 

Monnyrus  oxyrhynchus.     Zeit.  f.  Wiss.  ZooL,  Bd.  84,  1906. 

31.  Schneider,  C.     Lehrbuch  der  Histologic.     G.  Fischer,  Jena,  1904. 

32.  Strassburger,  E.     Ueber  Cytoplasmastrukturen,  Kern,  und  Zellteilung.     Jahr.  Wiss. 

Bot.,  Bd.  XX.X. 

33.  Studnitzka,  F.  K.     Ein  Beitragz.vergl.  Histologic  und  Histogenesis  des  Ruckenmarks. 

Sitz.  d.  Kgl.  Bohm.  Ges.  d.  Wiss.  in  Prag,  1895. 

34.  Wilson,  E.  B.     Protoplasmic  structure  in  the  eggs  of  echinoderms  and  some  other 

animals.     Journ.  Morph.,  Boston,  vol.  15,  supp.,  1900. 

35.  Hardy,  W.  B.     On  the  structure  of  cell  protoplasm.  Journ.  of  Physiol,  vol.  xxiv,   1899. 

36.  Dahlgren,  U.     Origin  of  the  electricity  tissues  of  fishes.     American  Naturalist,  vol. 

37.  Beard,  J.     Thetransient  Ganglion  Cells  and  their  Nerves  in  Raja  batis.     Anatomischer 

Anzeiger,  Bd.  7,  1899. 


EXPLANATIONS  OF  THE  PLATES. 
Plate  i. 

Fig.  I.  Transverse  section  through  body  of  9-day-old  embryo  of  Gymnarchus  in  region  C. 
L.M,  position  of  the  large  muscle  cells  that  will  eventually  change  into  the 
lower  median  electric  spindle;  V.F,  location  of  large  vertebral  fibers;  XXXX, 
locations  of  electric  spindles  in  an  adult  fish  with  reference  to  bony  structures, 
blood-vessels,  and  spinal  cord;  N,  canalis  centralis;  Bi  and  B2,  caudal 
blood-vessels.      X  no. 

Fig.  2.  Muscle-tissue  in  transverse  section  from  12-day-old  embryo  of  Gymnarchus  to 
show  typical  muscle  cells.  A,  one  of  oldest  fibers  with  two  strong  fibril 
bundles;  B,  growing  muscle  cell  with  fibrils  being  laid  down  in  a  ring;  C, 
extreme  upper  edge  of  myotome  with  youngest  cells  at  top.      X  1150. 

Fig.  3.  Transverse  section  through  group  of  larger  muscle  cells  found  at  L.M.  in  fig.  I. 
Under  greater  magnification.     For  explanations  see  text.      X  1150. 

Fig.  4.  Longitudinal  vertical  section  of  C  region  in  9-day-old  embryo,  ventral  portion. 
Between  marks  O  O  are  seen  same  parts  of  seven  myotomes  as  are  shown 
at  L.M.  in  preceding  figure  in  transverse  section.  Note  heavy  development 
of  these  cells.  A  line  drawn  between  marks  ©  ©  would  indicate  location  of 
seven  groups  of  vertebral  fibers  in  next  few  mediad  sections. 

Plate  2. 

Fig.  5.  Transverse  section  of  body  of  12-day-old  Gymnarchus  embryo  in  region  CCi.  D, 
dorsal  electric  spindle;  U.M,  upper  median  spindle;  L.M,  lower  median 
spindle;  V,  ventral  spindle;  Conn.T,  connective-tissue  septum;  N,  canalis 
centralis.     Bi  and  Bs,  caudal  blood-vessels.      X  no. 

Fig.    6.  Three  degenerating  muscle  cells  from  another  (3)  zone  in  fig.  12.      X  1440. 

Fig.  7.  Electric  spindle  showing  first  stage  where  electric  muscle  cells  can  be  called  an 
electroplax.  Complete  coalesence  has  taken  place.  Nuclei  are  in  peripheral 
layer.  Myofibrils  are  concentrated  in  middle  to  form  central  fibrillar  core  of 
structure.      X  1 150. 

Fig.  8.  Transverse  section  of  upper  dorsal  part  of  spinal  cord  in  a  Gymnarchus,  embryo 
9  days  old.  Neuroblasts,  in  region  that  will  later  show  electric  motor-cells, 
show  some  little  differentiation  and  growth.  Near,  and  above,  this  region  is 
one  of  the  transitory  giant  ganglion  cells.     X  940. 

Plate  3. 

Fig.  9.  Transverse  section  of  left  ventral  spindle  from  region  C  in  embryo  of  12  days.  To 
left  of  dotted  line  are  normal  muscle  cells,  to  right  are  degenerating  muscle 
cells,  in  the  midst  of  which,  and  surrounded  by  dotted  circle,  are  a  group  of 
muscle  cells  going  through  the  first  steps  of  transformation  into  electric 
tissue.     X  1 150. 

Fig.  10.  Transverse  section  of  ventral  electric  spindle  in  process  of  formation  in  region  B, 
of  12-day-old  embryo  of  Gymnarchus.  At  left  of  figure  two  muscle  cells 
begin  to  degenerate,  while  just  to  right  of  upper  one  of  this  pair  a  muscle  cell 
is  seen  in  advanced  stage  of  atrophy.  Connective-tissue  capsule  beginning 
to  form.     Electric  muscle  cells  rather  more  coalesced  than  in  fig.  9.      X  1 1 50. 

Fig.  II.  Transverse  section  of  upper  middle  electric  spindle  in  same  section  as  fig.  10. 
Rapidly  degenerating  muscle  cells  to  left.  Further  coalescence  of  electric 
muscle  cells  than  in  fig.  10.  Myofibrils  weakening  but  still  in  separate 
groups.      X  1150. 

Plate  4. 

Fig.  12.  Longitudinal  vertical  section  of  dorsal  part  of  region  D  of  12-day-old  embryo  of 
Gymnarchus,  showing  relations  of  muscle  tissue  and  electric  tissue.  Bracket 
I  indicates  zone  of  degenerating  muscle  fibers  on  inner  median  edge  of  five 
myotomes,  on  ventral  side  of  median  connective  tissue;  zone  3  shows  atrophy- 
ing muscle  fibers  on  inner  median  edge  of  myotomes,  dorsad  of  median  con- 
nective tissue;  zone  4,  rudiment  of  upper  median  electric  spindle;  zone  5, 
same  of  dorsal  electric  spindle;  zone  6,  muscle  tissue  from  ventral  spindle  to 
dorsal  outer  edge;  zone  7,  connective  tissue;  zone  8,  dorsal  epithelium. 
X  140. 

192 


Origin  of  Electric  Tissues  of  Gymnarchus  Niloticus  193 

Fig.  13.  Longitudinal  vertical  section  of  ventral  part  of  region  C  of  12-day-old  embryo  of 
Gymnarchus.  Zone  I  indicates  ventral  epithelium;  zone  2,  ventral  outer 
portions  of  five  myotomes;  zone  3,  lower  median  electric  spindle  which  is 
dividing  into  three  parts  in  five  vertebral  segments;  zone  4,  inner  degener- 
ating muscle  fibers  of  five  myotomes;  zone  5,  projected  outline  of  vertebral 
segments.      X  140. 

Fig.  14.  Motor  nerve  cells  from  spinal  cord  of  42-day-old  larva  of  Gymnarchus.  A,  B, 
and  C,  three  electric  motor  nerve  cells.  D  and  E,  two  muscle  motor  nerve 
cells.     From  region  E.     X  1500. 

Plate  5. 

Fig.  15.  More  highly  magnified  part  of  fig.  12.     Brackets  indicate  same  zones.     X  760. 

Fig.  16.  Highly  magnified  part  of  transverse  section  through  an  electroplax  from  C  region 
of  42-day-old  larva  of  Gymnarchus.  Curving  bundles  of  modified  myofibrils 
are  seen  in  core  and  cut  at  various  angles.  Outer  or  electric  layer  is  fixed  and 
stained  to  show  finer  outer  granules  or  electrochondria.  Vacuoles  as  well  as 
larger  meshes  of  the  linom  are  visible  on  inner  edge  of  electric  layer.  Elec- 
tric nerves  are  cut  in  sections  at  many  angles  and  show  medullary  or  connec- 
tive-tissue sheath  and  its  nuclei.  At  two  points  branching  of  these  nerve  fibers 
is  visible  and  at  four  points  its  contact  with  electric  layer  of  electroplax  is 
to  be  seen.     X  1500. 

Plate  6. 

Fig.  17.  Longitudinal  section  of  a  young  electroplax  from  B  region  of  12-day-old  embryo. 
Segmentation  of  electric  tissue  is  now  complete  into  unit  electroplaxes  of 
which  figure  is  good  example.  Central  fibril  bundle  beginning  to  lose  its 
transverse  striation.  Structure  is  as  long  as  three  muscle  segments  or  bony 
segments,  as  near-by  spinal  processes  show.  This  represents  oldest  stage  in 
embryo  of  this  age.      X  210. 

Fig.  18.  Longitudinal  section  of  young  electroplax  from  C  region  of  42-day-old  larva  of 
Gymnarchus.  Fibrils  have  lost  all  striation  and  become  waved  in  middle 
part.  Cytoplasm  shows  strong  differentiation  at  two  poles  (ends).  Anterior 
end  strongly  vacuolized.  Papillae  begin  to  grow  from  ends  of  middle  part 
and  nerve-endings  have  become  established  at  junction  of  posterior  third 
with  middle  third.  This  shows  youngest  stage  in  42-day-old  larva  and  is  a 
successively  later  step  in  differentiation  than  fig.  17.      X  210. 

Fig.  19.  Longitudinal  section  of  an  electroplax  from  E  region  of  42-day-old  larva  of  Gym- 
narchus. Fibral  bundle  much  "waved"  in  middle  third,  which  now  begins 
to  show  form  of  electroplax.  Papillae  more  developed.  Nucleus  and  some 
cytoplasm  inclosed  in  fibrillar  core.     X  210. 

Plate  7. 

Fig.  20.  Longitudinal  section  of  another  electroplax  from  further  caudad  in  E  region  of 
same  larva  of  Gymnarchus.  This  represents  oldest  stage  accessible  and  is 
regarded  as  almost  adult  in  its  histological  characteristics.  Two  nuclei 
shown  in  fibrillar  core;  one  of  these  shows  characteristic  swelling. 

Plate  8. 

Fig.  21.  Transverse  section  of  body  of  42-day-old  embryo  of  Gymnarchus  from  D  region. 
Greater  part  of  muscle  tissue  is  here  transformed  into  electric  tissue.  Section 
passes  through  very  last  part  of  dorsal  fin.  Electroplaxes  cut  at  different 
levels  and  in  one  case  section  passes  through  right  ventral  spindle  showing 
only  electric  connective  tissue.  D,  dorsal  spindles;  U.M,  upper  middle 
spindles;  L.M,  lower  middle  spindles;  V,  ventral  spindles;  Bi  and  Bi, 
caudal  blood-vessels;  N,  neural  canal.  Ni,  Ni,  Ns,  and  Nt,  electric  nerves; 
Ns,  lateral  line  nerve.     X  no. 

Fig.  22.  Part  of  transverse  section  from  middle  part  of  an  electroplax  in  E  region  of  same 
larva  of  Gymnarchus.  This  figure  serves  well  to  show  larger  granules  that 
lie  deeper  in  electric  layer,  also  connective-tissue  sheath  or  wall  of  electric 
spindle.     X  1500. 

Plate  9. 

Fig.  23.  Longitudinal  section  from  E  region,  showing  parts  or  whole  of  11  electroplaxes 
lying  in  dorsal,  upper  middle,  and  lower  middle  spindles.  Some  remaining 
muscle  fibers  are  visible  at  sides.  Figure  serves  to  show  that  there  is  no 
heavier,  transverse  connective-tissue  wall  lying  in  electric  connective  tissue, 
as  is  found  in  Mormyrus.  This  emphasizes  secondary  segmentation  found  in 
Gymnarchus.  X  no. 
13 


194         Papers  from  the  Marine  Biological  Laboratory  at  Tortugas. 

Fig.  24.  Small  part  from  edge  of  basal  part  of  a  papilla  on  posterior  surface  of  an  electroplax 
from  C  region  of  42-day-old  Gymnarchus.  Shows  a  single  nerve,  ending  in  a 
somewhat  elongate  and  branched  end-plate  in  electric  layer.  Outer  crowded 
mass  of  granules  stained  deeply  witheosin;  larger  inner  granules  not  stained, 
X  1500. 

Fig.  25.  Small  part  of  edge  of  another  part  of  anterior  surface  of  same  electroplax.  Shows 
a  nerve  fiber  dividing  and  each  branch  ending  in  typical  club-shaped  nerve 
ending  of  Gymnarchus.     X  2200. 


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